It should be on the LH side of the trunk behind the panel.

I would use the factory amp, makes life a hell of a lot easier instead of chopping up the wiring in the car. The factory amp gets it's "ON" signal from +12 V in the harness for the radio. I don't remember the color of the wire but it's there.

Hopefully you used the METRA wire interface harness- if you did, simply connect the amp on or antenna on lead to the same labeled wire in the harness.

I have an Alpine HU going to the factory amp in my car and an aftermarket sub powered off an Kenwood sub amp. Works great.

rjp

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03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)