Could be bad wire job, if you are not the origonal owner, maybe someone else did somthing weird? It would not be to much effort to run new wire from the doors to the head unit.
But the same thing happened to me. It turned out that the POS speaker that I put in to replace the stock one had a peizo (cheap!!!) tweeter in it. A few days after I put it in, it sounded *real* weak, and very crummy across the whole freq. spectrum. It turned out that the tweeter had a dead short in it, and since it was a peizo type, their was no capacitor crossover to help mitigate that failure. Bottom line, replaced the POS speakers w/ quality ones, and the problem went away! I found the dead short by using an ohm meter @ the head unit connector. Meausure both sides, and make sure they read the same. If they dont, your on the road to wireing, or speakers.