The button does not light up when pushed. I've looked, there are no obvious defects in the heater lines. But I don't know if it is the crappy tint causing it either. Any suggestions?

 

If there is no power to the button, check for a blown fuse. Congrats on 300K miles, I'm at 225K on my 1993 LX...

 

I would check for a blown fuse first. But, if the fuse is okay, I would suspect the rear defogger relay or the switch itself. See attached image for location of relay in your underdash fuse/relay box. I've seen where it turned out to be the relay.

You can test the relay and switch. There should be a service manual link in the DIY in this forum.

 

It turns out it was the fuse causing the problem. Thanks guys!