Help finding this wire

I'd just use a relay to convert the negative-polarity trunk-output (from your alarm) to the positive needed to pop your trunk.

An example of how to wire a SPDT or SPST relay:

30 and 86: fused battery power.

85: the alarm's negative alarm output for the trunk.

87: connect this to the output side of the factory trunk switch (the wire that gets power when the trunk button is pushed).

What will happen is that when 85 gets momentary ground from the alarm brain, 30 and 87 connect, sending battery power to your trunk popper.

Assuming no factory keyless or alarm (for simplicity), there is no negative trigger.

Most alarm trunk popper circuits will have a 200-500 mA negative output, which is only enough to energize a relay, not act as a power ground for the popper. Said another way, you need the relay anyways.

Wiring up the relay will take 5 mins.