Help wiring bathroom vent fan

I'm not sure what you're really working on but a fan is just a simple two wire device. I doesn't matter if you switch the ground or the hot wire. Either will shut the fan off. The problem you could run into (if you hook it up backwards) is the fans rotation being wrong causing the fan to blow air in and not out. With a voltmeter, figure out what wire is hot and what wire is ground. The fan may have a red and a black wire. Red goes to hot. Black to ground. If the fan wires are not marked, the wire to the switch is probably the hot one. Some pictures might help us understand your problem.