Glo plugs shorting out

[Lady Benz]

Hello and thank you in advance for any help here.

The glo plug light was not coming on in my 77 300D. My dad and I checked out the glo plugs to find that one was shorted out (not working -- Not sure which is the correct terminoligy) we replaced the plug (actually replaced all 5) and the light still would not come on. By accident we found that the fuse for the glo plug - not the big fuse but the one in the fuse box labeled caurberator heater had lots of corrosion. After we cleaned the fuse, everthing worked for about a week.

This morning - same thing - glo light not coming on. After checking glo plugs we found two plugs shorted out again.

Thing is, I had a mechanic working on it - he said when he was working on it, the element that attaches to the plugs was staying hot when it should not. He was stumped. He did not even try to fix it after I just spent $2000.00 for some work he did for me. Guess what? not going back there again.

My question is-- If the timer in the relay was to get stuck in the on position some how, would that be why the glo plugs are shorting out. Or is there something else that can cause this.

Sorry for such a long message but I wanted to give you as much info as possible.

Thanks!! Cendy