See, a pinch or a short in the RCA cable could cause the internal amp to shut off as a protect for the whole cd player. If they are plugged into the amp you were trying to get to work and there is a internal problem with that amp. You have the possibility of the RCA cable being shorted by the amp, despite the fact the power wire is not hooked up, because the ground wire is still grounding that amp.

If I were you I would start by disconnecting the RCA cable and remote wire from the amp(put electrical tape on the bare wire). Then reset the radio by pulling the radio fuse. I have seen Panasonic radios with reset buttons before so you may have one. If the radio does not give any sound to the inside speakers at this point you still have the possibility of a wiring issue with the speaker wires. If any of the wires came loose and are now grounding(ie touching the cars body or + and - wires touching), it would cause the amp to stop working until that short is fixed. So check all the speakers for loose connections and maybe have the shop check their work. Loose connections can cause this same issue. Also if you speakers are old and are damaged internally, they can cause a short and as a result cause the internal amp in the radio to turn off. These are all the possibilities I can think of so if your willing to check this stuff you might find your problem. As for the radio, if you bought the radio new you still have warranty so look into that possibility.