Rear defrost button lights up on dash when pressed and looks like everything is connected at the glass end. Where is the defrost relay at? Thats the only thing I can think of that could be bad. Would the light still light up if the relay was bad?

Although I don't have the 1988 diagram , I believe this 1992 diagram to be the same . With that said , the light appears to be controlled by the relay's switched contact so if the light is lighting the relay should be ok . Were this mine to diagnose I'd start by turning the defroster on and checking for +12V on the left (driver's) side strut connector . If it's got power there then I'd check the right (passenger's) side strut connector to make sure it's grounded .

In addition to what Orangebird said, I have seen where over time the lines get damaged to the point where current can't travel across them and more. Way back in the day I found a kit that was sort of like white out and had a stencil to repair the broken lines.