Astro Heater /Blower Speeds?

[AkDale]

Hi all,
I have a 1990 Astro AWD. The heater blower used to be intermittent. (Sometimes I had to smack the passenger side dash to get it started). When it did start, it would only start while the selectors were on DEFROST and the fan speed on HI. If I changed either control, the blower would shut down. It recently quit completely.
I asked the Chevy dealer-- They sold me a new set of resistors and relay. This did not solve the problem. The switches appear to be working correctly.
Can someone explain how this system works, and maybe take a stab at a diagnosis?

Thanks,
Dale

[larryboy]

my friends 91 safari was the sliding switch on the dash,same symptoms.

my 92 safari was the heater motor itself,similar symptoms.