This is a quickie for a friend with an '80 model. 

no doubt a Joel question to answer, since he's got the wiring diagrams, but.. logically thinking(dangerous i know, considering all the great engineers that GM hires),  the rear window defrost should be on a circuit to itself.. and as for the heater, do you mean the fan won't come on??.. if so, then, it too is on a seperate circuit. The raising of the headlights are operated by vacuum... there is a valve on the headlight switch that provides the vaccuum to the actuators once the pull switch is pulled all the way back... it is possible that the valve is shot, or that the vaccuum hoses are just dry rotted and leaky.... however if it is the hoses, then its not the switch...  kinda sounds to me like 'things' are not as they seem where-ever this vette is being cared for.... but then again i'm not known for my worldly intuition.... :) 

A bad headlight switch can keep the lights from raising, but only due to a vacuum issue with the switch, not electrical. If the headlights actually light up, then the electrical part of the switch is working. The switch can dry out, and cause the lights to not raise, as the headlight vacuum signal does go thru the headlight switch.
The headlight switch ain't got dick to do with the rear defog, or the blower motor. Those things have different problems, possibly as simple as a fuse...
I can e-mail you some vac. troubleshooting info...I shall do that now.
I would tell yer buddy to get his car out of there...


Mike..."You've Got Mail!!"
(MS Word Doc)Adams' Apple2008-09-03 19:07:59

Thanks all.  You did not dissapoint!  And I especially appreciate the Vacuum Troubleshooting Guide that Joel sent!