Have a '79 300 SD. Can now only get cool air. Previously had a cracked servo unit and could only get hot air. Replaced a bad aux heater water pump that was bad. Have put in two different used servo units from junk yards, but neither are able to switch to hot air. Is there a way to determine (for an absolute novice) if the temperature dial is working? At this time of year hot air would be better. Any way to manually change the servo to a heat setting? Any easy way to determine if the servo units are ok? Help!!