If your heater control setting only runs on low or no speed settings,replace resistor:
Heavy blower resistor (Toy EPC calls it a cooler/coil resistor,fan coil resistor) for fan # 88635-89103 (85-86 4-Runner,4 speed fan) & 87138-89105 (89 pickup)
http://www.4crawler.com/4x4/CheapTricks/Blower.shtml
Sidney