ok, some progress made

i got all 4 speakers working very quietly at max volume. no idea how

the loose blue wire mentioned above kills the sound if it touches the ground or the stereo chassis and you have to turn the ignition off and on to get the quiet sound back

the blue wire that this loose wire is spliced off is labelled "power antenna". my vehicle has a manual power antenna working off a dash switch up/down control and it has never been auto connected to the radio as long as I have owned it, but I assume the previous owner had it hooked up at some point

the radio chassis is labelled negative ground.