[Cunningr]

So your rear speakers are in the shelf behind you? Just guessing since you have an aftermarket hu, that those are being powered off the hu and possibly whoever did the hu couldn't figure out how to wire in the amp in the frunk. Normally the amp in the frunk is a m490 which has 4 outputs 2-to dash 2 to doors. The door channels are only low frequency though.

Amp works ok, but i swapped in a 6 channel 996 cabriolet with stock hu then went aftermarket. Speakers in dash disintegrated on mine.

Anyway the stock radio has control out and line levels to the frunk amp.

Google m490 porsche wiring diagram should get plenty of hits with wire colors. I didnt chop up my wiring harness so cant help there. Might double check the porsche code too for you sound package