Another question is "has the radio been refurbished and then replaced in the dash"? I had a 91 that I had MAMW refurbish [they sent it to Texas] and when it was returned and re-installed professionally, the radio unit would not turn off. Since the unit was sealed, the professional installer (and a local refurbisher next door who confirmed it would not turns off) would not open the seal to find out what was wrong as it would void the warranty. MAMW had me send it back at no cost and their refurbisher in Texas found that one of the pins in the power wire was bent and this prevented the unit from shutting off. Was repaired, then returned and installed. Worked fine. This may not be the problem, but it could be. Hope this helps.
Barrett