ranger problem

[bloodhound]

Help--I'm a new member and seek guidance...last month I bought a '99 Ranger 4x4, and problems now surface, almost all to do with Heater/blower....The fan either goes on high or not at all. How hard is it to find/replace rheostat.......(2) last night the defrost (we live in Maine) was in high fan mode, when, suddenly, the air temp.went from warm to cold, the change occurring in step with red stop engine light coming on; this was then followed by the smell of burning rubber (wiring insulation odor).
The fuses checked out OK, fine......The next morning everything back to preincident order...heater core problem? Wiring problem? Where to look first, last?
Thanks in advance for any leads.