My experience has been that if you can verify cold air by doing what you have done, may as well shop for a push button panel. Sure it COULD be an open in one of the wires. That's the purpose of the test that I gave you, but you'd need a voltmeter, which I believe you could go out and buy relatively cheap at an electronics store or even Sears. But ultimately, I feel 95% certain that your looking at a bad pushbutton panel/control unit. You can check out this website, Fastlane, or call your local MB dealer for a price, unless you're really electronically savvy, it's no use messing with it.
Gilly