Did finally have my original go out at about 110k miles, but the replacement worked well for another 65k and no problem. But it does sound like your replacements are junk. With that one falling apart, it sure sounds like it anyway!
I don't remember if mine was a Motorcraft or not, but back then it might have been, or the switches were just of better quality 20+ years ago.
Both the suggestions from Steve83 seem like a good next step at this point.
Good luck. What you're experiencing is pretty extreme.
Hmm, something else to think about here. When you bolt them down to the floor, is the floor warped where it mounts? Maybe the stress of bolting to a crooked panel is overtaxing it's little body. Worth looking at anyway.
Paul