Fan Speed

[84GP455]

As far as your blower fan only running on high speed, that is your blower motor resistor pictured here http://www.rockauto.com/catalog/moreinf ... cc=1255182
It's under your hood near the a/c components with a 3 prong plug hooked up to it.

right here^^^^

 

[GT_80]

did you make sure the fan switch has continuity on all terminals? it could be the switch itself is burned out, and everything else works. In my experience, there are like 4 brass contacts on the switch- one or several ofthose could have gotten corroded or burnt out over the years. Just something else to check.