Do not convert to 134a unless you HAVE to. Nothing is as good as R12. The system was designed for R12 and ANYTHING else will not be as efficient. The substitutes work half-arsed but the original R12 system is the best and will run longer without component failures.
Absolutely sounds like the switch/fan. Like someone said, if the fan is not running the a/c system will not work. I'm sure you noticed in the past, that unless the fan is in one of the ON positions, the a/c light is not on...