I just bought a used alternator on eBay for $20. It was a rebuild and obviously has very few miles on it. You can look for such deals there or at your local salvage yard, or you can pull the voltage regulator out of the back of your alternator and just replace that, which may solve its issues. The VR is under $30 I believe at www.fcpgroton.com or www. eeuroparts.com or you local foreign car part store. The Suby alternator will not likely bolt into your 240, maybe one from another euro car (if German Bosch) but not from a Japanese car. You sure the problem is actually the radio, or an ignition switch problem? How did you come to the conclusion that it was the radio? That seems very unusual.
Charles