Adding sensor to factory alarm

I am trying to add a DEI 504D shock sensor to my factory alarm on my 2013 Subaru BRZ.

I am trying to use the trunk sensor\trigger. On the trunk latch/release mechanism, there is a plug with 3 wires in it:

brown - remote key fob trunk opener; gets +12v when the remote trunk opener is used
black - ground
BLACK/ white - Unsure how this one works. When the plug is removed from the assembly, this is 12v+ and the trunk light turns off. If I ground this wire, the trunk light turns on. When the alarm is armed and this wire is grounded, the alarm goes off. With that said, I've attempted to use this wire with the sensor. The light turns on in the trunk when the sensor senses a shock so my wiring is correct as I planned it, but the alarm does not trigger. I'm not sure what I'm missing here. Can anyone assist?

Probably won't work; this has been tried before many times, it's to do with the signal sent out not being compatible with the OEM alarm's sensor input.
Failed years ago ago trying this with a glass break sensor on a Civic.
Yes, the BLACK/ white is correct.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

But how would it work when I manually ground the wire to the frame of the car then?

Is there a chance it would work if I used a relay to give the alarm a ground... Using the sensor as a trigger to the relay?

I think you might be on the right path, worth a try.
At the time I never thought of boosting the current with a relay.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Thanks for the replies, I'm going to give it a try and I'll report back whether or not it works.

I'm no electronics guru, just want to confirm that it is normal/ok/safe to ground a wire when it has 12v+. It seems normal as it makes my trunk light turn on, but isn't that shorting out the wire which is bad? Or is this normal?

If the bulb is left in, that BLACK/ white will show 12V+ until it's grounded.
Here's your circuit:-trunk_light.png

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Using a relay to ground the trunk wire works perfectly. The trunk light also now comes on and off immediately, where when directly linked to the sensor it would dim on then dim off. Now, it's like a light switch when controlled by the relay.

Thank you for the help!

Thanks I think you've solved a long running problem.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Had the same issue,thanks for posting.

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Do it once,do it right - This means no short cuts. You never get paid twice for having to do the job twice because it wasn't done right the first time.

For those that are going to attempt this, I used a SPDT relay from Radio Shack, p/n 275-0031, which has a 30mA coil which the sensor is strong enough to energize.

Thanks for the info buddy!

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Do it once,do it right - This means no short cuts. You never get paid twice for having to do the job twice because it wasn't done right the first time.

Good Job, I will definitely try this. Just an additional tip, you can isolate your car system from your add on circuit by using a diode, just to be sure that the add on circuit won’t affect the rest of the circuit.

Regards,
Sevgi S.

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Good thinking, I had a sseesenior moment, that's a given.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.