I'm kind of leaning towards PIC for this. My past experiences with a few 555's has left a bad taste in my mouth. (I guess I might be expecting too much from them, but I've never managed to build anything with a 555 that worked properly for very long. But then, I don't really ever know what I'm doing.)
My problem with using a PIC is that I've never used one before. Oh well, now is as good a time as any to start.
I think I'll start researching. Anyone care to suggest some good resources?
The little black box known as a BCM controlled the defroster circuit. I dismantled one to see what made it tick, and there were no 555's in there at all. Just a lot of SMD stuff that prompted me to buy a bigger magnifying glass.
As far as sensing the opacity of the glass is concerned, that's a pretty neat idea, but I'd like to try to keep everything as simple and compact as possible. Plus, I don't think anything like that would be possible without mounting stuff on the outside of the car, and that just violates my sense of aesthetics.
The idea could come in handy at some point in the future though, so I'm going to file it away with all of the other stuff that I keep in my brain.