1987 ford ltd crown vic w/ premium sound

with five wires at the output of the radio, or to the input of the amp, the factory radio is common ground.  your aftermarket headunit may not be, most aren't these days.  you'd probably be best in bypassing the premium sound amplifier.  have to remember where the amp is on that model...either trunk or center of dash panel, at the top.

m

beat me to it...sounds like the only wires in speaker harness is the + wires...well, sounds like its time to run some wire...which is probably not a horrible thing...you can use a fairly small wire...might as well look into upgrading speakers why you are in there...

It looks like the WHITE/ Red wire is a larger wire that the other 4.  Is this so?  If it is larger than the other 4, chances are it is the ground and the others are the positives for the 4 channels.  Now that you have the amp turning on, it should be pretty easy to figure it out.  Using a 1.5Volt battery, AA AAA C or D battery and a short length of speaker wire, we can figure it out.  Strip both ends of both the positive and negative speaker wire, insert one end into the harness, one on the larger wire (If it is larger).  Turn the ignition on and turn on the aftermarket radio.  Unplug the aftermarket radio from the speaker harness.  Now touch and hold one wire to one terminal of the battery, now touch the other wire to the other end of the battery, it should make a speaker pop.  Notice which one pops.  Label that wire as that speaker.  Repeat for the other 3 wires.  If  all of the wires are the same size, get some help and stick the wires into any 2 of the slots of the harness that are occupied with a wire.  Touch to the battery, if more than 1 speaker pops, you have the positive for those 2 speakers.  Once you figure out which wire is the ground for all 4 inputs, do not connect it to any negative speaker wire.  Ground it to ground.  Insulate all speaker negative wires of the radio and do not connect any of them to anything.   Let me know what you find out.

Sorry Tommy,  I have not encountered one of those vehicles.  They have done many of them here, they all whine about having to run wires to the speakers.  If you are trying to get the headphones working, and you know which wires are for the headphones,   You can do it kind of the same way.  Only difference is you need to install a 1 microfarad non-polarized capacitor in series with the positive speaker wires.  And ground the common wire going to the headphone jack.  As far as the speakers, they will work like this but you will need a much larger capacitor.  Do not connect the radios negative wires to anything, insulate them and leave them disconnected.

He has an amp with a common ground input.  You have speakers with a common ground setup.

plethora of knowledge...as always...Thank You for making my life easier...