OK, I picked up one of these buzzers at Radio Shack today and just spent the last 20 minutes or so installing it. That time included gathering up everything I needed and putting everything away when I was done. I mounted the buzzer outside the fuse box with some double sided 3M tape, securing it to the adjacent body panel.

I really like how it isn't terribly loud and annoying, just enough to confirm that the doors are locked/unlocked. Once again a big THANK YOU to corfire for the how-to on this!