RLB6

Installed in an Audi A4. front speakers are powered by the internal amp, rear and subwoofer are getting their signals from the RCA pre-amps and still using the factory amp. While on the road, my front speakers just quit working and only my rear speakers and subwoofer continued to work. I reset the unit twice but still no sound to the front speakers. Any Ideas as to what happened? Is this is known problem with a easy fix?