A previous owner of my 81 Conquista replaced the original AM/FM with a Phillips AM/FM cassette deck. The radio turns on, but the sound is so low as to be useless (it can barely be heard when turned all the way on, primarily you just hear the speakers click when turning the unit on and off). I've replaced the internal .5 amp fuse on the Phillips, but it made no difference. I can see no loose or disconnected wires.

Any suggestions? I do have the original AM/FM that came with the truck, and I'm debating wether or not to just re-install it as I would eventually like to restore it to stock condition.

If you're sure the speakers are good the outputs of the stereo could be blown or it could need an external amplifier, just guessing. Some stereos do require an amplifier or amplified speakers.

I know it is not much help.

Evening Jim. Any chance you can just hang the old original radio to the side and see if the wires will reach? Then you can try the old radio and tell if the Phillips radio is bad.
Also, is the "low sound" just on the radio or when you play a cassette?

No luckon the cassette - no sound on that either. There's not enough wire left on the original AM/FM to reach - whoever cut it, only left me about 1/2 inch. I'm going to play around with it this weekend, and if I can't get the Phillips to work, I'll just pull it and replace with the original, which is my ultimate goal anyway.