Problem with Sony Deck in 98 Jeep GC

I installed a Sony CDX-F7700 deck in my friend's 1998 Jeep Grand Cherokee today. I used a standard wiring harness and soldered everything. After installation, the deck would read a cd, show it was playing but no sound would come out of the speakers. However, when turned to the radio, the sound was fine and everything worked normally. AFAIK the jeeps don't have an internal amp, and even if they did the fact that it worked fine on radio made no sense. So, they took the unit back to best buy and got another one, and the same thing happened. Did I overlook something when installing the deck? Thanks! Chris

I'm an idiot...what the problem was is the harness we got from best buy called for a power antenna hookup, however it's actually the amp turnon. when i had it on tuner, it would send power to the power antenna wire, which would in turn turn on the factory amp. I switched it to the amp turn on wire from the new head unit and it works fine now. -Chris