Rear Window Defroster problems

[wastintime]

Hey everyone,
Well, it's getting cold enough in the mornings even in SC that I'll have to use the defroster every now and then to see out of my rear windshield. It seems to be working horribly though. The car is a '92 C2 and i know the rear glass has been replaced at some point. When I redid the speakers in the back I made sure that everything was properly connected as well.
However, everytime I turn the defroster on only the middle 4 bars or so work. I'd been told that older 911s had a two stage defroster button, where the first detent only allowed the middle bars to work, but mine doesn't seem to have that. The few bars that do work don't seem to work very well at all. This seems fairly adnormal to me, can anyone think of anything that might be causing this?

Thanks,
Andrew