I'd say you shorted some wires when you cut them and blew a fuse. I don't know what circuit the tach and voltmeter are on, but is could very well be the window circuit (I'm too lazy to look it up right now, but can later if you need it).
Mine also works just fine when I take the doors off, but just unplugging them won't effect it unless the wires short out and a fuse blows.
Tape off any cut wires, or just unplug them from the kickpanels :doh: , and check the fuses (there's some under the hood too).