I have an 86 z-28. If I turn on the rear defrost, it works fine, but will only stay on for maybe 30 seconds or so. What would cause this? I have to turn it on about ten times before the back glass is clear. Any help would be appreciated!

 

check the connectors on the trunk lift struts and see if they come loose or get a hayes manual and look at the wiring diagram and see if you have any ffayed wires or loose connectors