i didnt read the whole story but from the looks of it you just used the knob of the original radio to turn the sound louder or softer , if theres a guy here thats an electronic wizard or at least knowlegable about the cirquits on the original (working) radio he might be able to point out where to cut the original tracks place resistors or other stuff to modify the signal so it dont overload the system and solder the wires for the rca's on directly so the signal flows directly intoo the original preamp of the old radio , you could then tap the signal of the cirquitboard right after the preamp not using the mainamp from the radio so the radio/tapeplayer can allso be used through the big amps and speakers
a smal switch behind on of the presetsknobs can switch between the outside and inside signal or insert an old tape to select it / silence the radiosignal, it would save you some hacking and the potmeters
theres a whole website about these sort of hacks along with picking locks and the sort , its
http://hackaday.com/ if you guys are interested .... its pretty thechnical at times though