I know this isn't a new subject, and while I can find plenty on the location of the speakers I can't find anything on the wiring.

I have an aftermarket head unit with the adapter still using the stock harness.

Will it work if I wire in the speakers to the new head unit or am I going to have problems since the car never came with rear speakers in the stock harness?

 

i have been looking for days and many hours. Lot of "experts" giving advice, but nobody seems to know how to simply connect rear speakers to the stock radio.

 

here you go. Just tap some speaker wires into these wires and you will have rear working speakers.

Left Front Speaker Wire (+): Green/Yellow
Left Front Speaker Wire (-): Gray/Red
Right Front Speaker Wire (+): Green/Black
Right Front Speaker Wire (-): Light Green

 

The headunit itself has rear channels, it's just that there is no wiring in the AP1 harness
to access them. The right way to do it is to add four wires/pins to the harness connector.
This can be done by taking them from another connector - my guess is a Metra 71-1721
would work; using the factory solution documented in this thread; or apparently using pins
from as ATX computer power supply as documented here.