Part time 4 wheel drive light

[phatton]

The part time 4 wheel drive light in my 90 Comanche seems to be tied into my a/c, vent, heat and defrost switch.  In 2 wheel drive, when I turn the switch to the off position the part time 4 wheel drive light comes on.  If I place the switch in any other position the light goes off.  The light does come on when I shift into 4 wheel drive.  I believe that the heat and defrost run off engine vacuum  - could I have a common vacuum line going to both the switch and the light?  How do I tell?  Any ideas?  Thanks

[JMO413]

This is the switch that's turns the light on.

It's on the passenger side inner fender very close to the firewall. 

[phatton]

Thanks