Aftermarket headunit install

[mnsilverado]

I helped a friend install an aftermarket head unit in his 4-door 95 Tahoe the other day and now the rear door speakers don't work.  (Only the front doors and the rear ceiling speakers work)  I pulled the unit back out and noticed that the wire harness adapter does not allow the aftermarket unit to connect to two other wires on the factory plug... I am guessing that those are what run the rear door speakers. My question is.... Is there an easy way to get the rear door speakers running off the aftermarket unit while using the adapting harness?  Thanks