Nissan Altima Radio Problems

I have been having probleems with my daughters' Altima OEM HU (Clarion CD/Cass/Tuner). It would work (for a while) only if I pulled it out and unplugged/replugged everything.

I bought a new HU with wiring and antenna adapters, and installed that. Everything worked(power on, equalizer,memory,etc) except I got no sound. I pulled out both amps in the rear deck to inspect them. When I ground the amp for the rear speakers (reinstall it in its holder), the rear speakers come on. Then, when I ground the front amp, everything dies (grounding it gives a pretty good spark). If I unground the front amp, the rears come back on.

Do I have a short in the front speaker wiring? I swapped amps, just to see if the problem was with them, but I get exactly the same result.