99 mustang, bad amp?

ok, so i have a 99 mustang with the stock premium mach 460 sound system.  i decided to add an amp and two subs and found that it was much more difficult hooking it up with the stock system than it should have been and i finally finished it.  however, due to the fact the HU had no RCAs i had to splice into wires in the rear amps that came stock to get a high level input signal.  the problem is while doing this i discovered one of the two stock amps i would have to splice into is not functional at all, it's delivering no signal to anything.  i figured this was something that i could live with and moved on but now that i have finished i have discovered that the channel (which happens to be the left one) that comes from the wires run through this amp just makes an awful humming noise regardless of what sub i hook up to it or anything.  so do i just need to replace this amp or is it something more causing the problem?  thank you in advance for all help and advice!

well, there are quite a few problems associated with mixing factory and aftermarket equipment, especilly factory amps. so your saying that the factory amp wasnt reproducing sound at all? before you put the subs in? it could be bad, lost a wire, etc.   my suggestion to you is to scrap the factory head unit, and get rid of the HO hookup in favor of the RCA's on most moder car head units, and have the speakers run of the factory amps, or diconnect the factory amps and just run pure head unit, OR replace your factory amps---if the amp is messed up, that is.

that's correct the factory amp reproducing any sound at all and when using the channel that amp's wires should have been supplying a signal to it just produced a loud constant humming bass noise even if there was no music playing at the time.  i know it would be easier and probably more practical to just by a new HU but i would rather not go that route if at all possible.