This comes up often in our forums and I've been meaning to address this for some time. If you have a vehicle that has had the factory stereo connector cut off from the stereo harness and do not know what each wire is in the factory stereo harness, you can follow these simple steps to determine what each wire is using a digital muti-meter and a AA battery or with just an analog meter. You may also need a 1 amp fuse and a few feet of 18-20 gauge primary wire.
First, with the ignition switch in the off position, test each lead for 12V+. If you are using an analog meter (my preference for this), set it to read DC voltage at or above 15 volts. Setting your meter below this could cause damage to your meter or at least blow an internal fuse. Attach the common lead to a known ground in the vehicle and probe each wire with the positive lead. Make sure you measure every lead in the harness even after you have found one that shows 12V + and record your findings. Typically you will only have one 12V+ constant wire in the harness, but some vehicles may have more. This will connect to the memory lead of your head unit, normally yellow on most aftermarket head units.
Second, with the ignition switch in the off position, turn on the vehicle's parking lights and make sure the dimmer is turned all the way up, then probe every wire in the harness again that did not show 12V+ in the first step and record your findings. When you have found a wire that shows 12V+, leave your meter attached to the wire and adjust the dimmer to see if there is a change in voltage, then continue to measure the remaining leads. The lead that shows 12V+ with the parking lights on that does not show a change in voltage when you adjust the dimmer switch is the illumination lead. The lead that shows a change in voltage when you adjust the dimmer switch is the dimmer wire. Your head unit may or may not have a corresponding lead for either of these (typically orange and/or orange with white stripe on most aftermarket head units).
Third, with the ignition switch in the accessory position, measure each lead. You should find one that now shows 12V+ only when the key is in the accessory and run positions. This is the 12V+ switched lead or accessory lead and will connect to the red lead of most aftermarket head units.
Fourth, with the ignition switch in the off position, set your meter to read resistance at the lowest scale and zero out your meter if you are using an analog meter. Reconnect the common lead to a known vehicle ground and probe every lead that did not show 12V+ in the previous steps to find the ground lead in the harness. This will connect to the black lead on most aftermarket head units.
Fifth, turn the ignition switch to the accessory position and momentarily touch each lead to ground that did not measure 12V+ in the previous steps. The purpose of this is to find the negative power antenna trigger lead if your vehicle is so equipped. Keep in mind, most vehicles do not have a negative power antenna trigger lead, so if you do not find one, don't be alarmed.
Sixth, if you are using an analog meter, set it to measure resistance at the lowest scale and connect the common lead to any of the remaining leads that did not show 12V+ or ground in the previous steps. If you are using a DMM, set it aside and connect one side of the AA battery to any of these leads. Now connect each of the other leads, one at a time, to the positive lead of your analog meter or to the other side of the battery until you hear a popping sound from any of the speakers paying close attention to which speak is popping. If you can see the speaker when it is popping, pay close attention to the direction of cone travel. If it moves outward, you have the correct polarity for that speaker. When you have found a speaker, record the two leads for the respective speaker and set them aside. Continue to measure the others in the same manner until all speaker wires have been identified. If you are unable to locate any speakers in this step, you most likely have an amplifier. If you do not have an amplifier and have found all the speaker leads, but can not see them to confirm the correct polarity, don't worry, you can check this during the installation of the head unit.
Seventh, if you have any leads not yet identified after performing all the previous steps and you have identified all speaker leads in the previous step, the only lead remaining that you may need is a positive power antenna trigger if the vehicle is equipped with a power antenna. If it does not have a power antenna lead, you can proceed to connect your head unit. If the vehicle does have a power antenna and you have not identified the power antenna trigger lead, you will want to visually identify all leads at the power antenna relay or at the power antenna itself. If you are working on an older vehicle that does not depend on the head unit for other vehicle functions and you can not visually identify the power antenna leads, you can try connecting a small gauge wire with a one amp fuse in line to the 12V+ lead you found in step one and momentarily touch it to the remaining unidentified leads while watching the power antenna. If you find one that causes the antenna to go up while applying 12V+ to it and the antenna goes down when you remove it, write it down and proceed to install your head unit. If your vehicle does depend on the factory head unit for other features in the vehicle, post your request for this information in our forums.
Eighth, connect all respective power and ground leads you found in steps one through five and step seven. If you have identified the correct polarity of each pair of speaker leads, connect them too. If you were not able to correctly identify the correct polarity of each pair of speaker leads, connect only one speaker and make sure the other speaker leads from the head unit are temporarily insulated. Turn on the head unit and adjust the volume to a low to moderate level and listen for a bit, then turn off the head unit. Temporarily connect another speaker to the head unit and turn it on and listen again. If the bass increases, the polarity matches that of the first speaker you connected. If the bass decreases after you connect the second speaker, reverse the two leads going to the second speaker. Continue this process until all speakers are connected. If you were able to visually determine the correct polarity of any speaker, but not all of them, start by connecting any of these first, then follow this process to connect the others. -------------------------------
If your vehicle has a power antenna and requires a ground to activate it, connect an SPDT relay as follows:
30 to ground
85 to ground
86 to power antenna output lead of your head unit (usually a solid blue wire)
87 to negative antenna lead
If your vehicle does have a factory amplifier, connecting your head unit to the factory amplifier will vary depending on the vehicle and type of factory system installed. In some cases the amplifier input leads will be the same respective colors as the speaker leads in the same vehicle without an amplifier and may be connected directly to an aftermarket head unit, but will require an additional connection to turn the amplifier on. Typically this wire needs to see 12V+ when the head unit is turned on. The remote turn on lead is usually a blue with white stripe wire on most aftermarket head units and can be connected directly to the factory amplifier's turn on lead.
In some cases, the factory amplifier turn on lead requires a ground to turn the amplifier on. If this is the case, connect an SPDT relay as follows:
30 to ground
85 to ground
86 to remote turn on lead from your head unit (usually a blue with white stripe wire on most aftermarket head units)
87 to negative amplifier turn on lead
In some cases, the factory amplifier will require some type of interface and can not be connected as described above. If this is the case, please search our forums or post a request for additional information.
If I've overlooked anything, please feel free to add to this topic.
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