no audio from radio; 92 camry wagon
 

about 5 months ago my radio stopped producing sound; it still lit up and was fully functional except for the lack of audio. or so i thought, now i've finally boght a new head-unit. when i went to plug this new unit in i experianced the same issue, no audio whatsoever.

the first head-unit (that i thought was bad) is a pioneer deh-4600mp. the new HU is a pioneer deh-5000ub.

since neither of them are outputting sound i assume it's a problem with the cars internal amp (the wiring diagram says that there's one) i've checked the fuses multiple times, all the wires that i have easy access to are intact. does anyone have an idea of what could cause this? my best guess at this point is the supposed internal amp. is it located in an accesible spot? how can i test it? what will happen next episode?