So the front defroster of my vehicle seems to work by blowing air up at the window. The rear defroster seems to work having a bunch of wires that run the length of the window. My question is... would there be any harm in just leaving the rear defroster on constantly?
Well, there are some downsides. First, most cars turn it off automatically so the manufacturer doesn't think you should leave it on all the time. The way the defroster works, it's a grid of thin conductive silver ceramic lines that are painted onto the rear window, before being baked on at high heat. In operation it is a high current device that on average heats to roughly 85°F (29°C). Something that draws a lot of current all the time is going to put a strain on your alternator and potentially your lights will dim at low RPMs as the voltage sags from the load. I would not recommend letting it run all the time as that defroster is placing a lot of unneeded load on your electrical system, and the defroster itself. If it's too warm, the unit may wear out a lot faster than it would otherwise. It wasn't designed to be run all the time, hence the auto shutoff, so I wouldn't force it.
Hope that helps!
I've seen GMs that burn up/melt the passenger side relay block/trunk fuse panel. Due to the rear defroster being run constantly.
If you leave it on while the engine is not running, then you will quickly flatten the battery. Other than that, it will do no harm leaving it on.
It will of course cause a small increase in fuel consumption.