Good morning everyone. I hope that someone here has an answer to my problem as this has become very frustrating!

This past weekend I received a a F700BT from a friend as a payment for some work I did for him. The headunit worked and didn't have any issues that we knew of, but it had been sitting on a shelf for about 6 months. 

I installed it in my 2003 Avalanche with a GMOS-04 Adapter using the factory Bose system. I do have a pioneer 12" sub in the back, hooked up via the sub outputs. 

Whenever I'm on AM/FM, it works and plays sound throughout the system, including the Bose speakers. 

However, whenever I switch to Video/Aux/SD there is no sound except for the sub. I tried hooking up an amp to the Front/Rear RCA outputs and those do work. How can I get the Head unit to push sound through the factory harness instead of RCA outputs? I'd rather not have to go and replace all the speakers to amp them. 

Thanks!