Originally Posted by Brian-bmw

The factory speakers terminate at the amp in the trunk. If you are installing an aftermarket amp and aftermarket HU, I assume that you are also replacing the speakers too. You can still use the factory speaker wiring without issue. If you are replacing speakers, they probably come with passive crossovers, so I would install those in the trunk with the amp and then use all of the factory direct runs to the individual speakers.

Additionally, you could even re-use the factory audio lines that run from the radio in the dash to the factory amp, if you got the right adapter for the dash radio plug.