My rear window defroster only comes on for a bout 30 seconds then it kicks off. So it stops just as it is starting to work. Then after it goes off it will not come back on for atleast 2 min and, then shuts off after like 5 seconds. This is in a 93 t/a . Any Ideas ?

There is probably a relay that is getting weak or there is to much resistance in the wires and is overheating them, shutting the relay off. Either way I would look at the relay. I am not familiar with the 4th gens, so I don't know where to look for it. Probably under the hood. Before you go out and buy a new one look for another relay that is the same and swap them. If it works then there is your problem.:D