D150 wiring issues, help!

A basic radio will be no problem. They don't draw much juice. All it takes is a loose wire on the ammeter post and it'll start to arc, overheat, and melt the housing. It's hard to say for certain which came first. It's kind of the chicken and the egg argument........was it a loose wire that caused the overheating/melting, or, did it overheat a bit causing the wire to loosen. With plastic housings it could be either way. With my '78 Powerwagon, I use the '72-3 pickup housing, which is pot metal. No problem with those melting or warping.