I was getting alternator whine, so I decided to see if swapping my Kenwood HU for a Clarion HU would help at all. Now after I had plugged it in and turned it on, the display came on fine, but as I was reaching for a CD to test it with, the HU just turned off by itself.
I noticed that it would still accept the CD and eject it, but refused to turn on and play it. I then took the Clarion HU out and put the Kenwood HU one back in, same thing: accepts the CD and ejects it, but refused to turn on and play it. Then about an hour later it refused to even do that.
The on-board 10A fuses for both these HU's are fine, obviously the HU's aren't the problem, and I wonder if the key to figuring out this problem is the fact that it would still accept and eject CDs even though nothing else was working.