I am installing a bathroom vent/light unit.

Currently there is only one light abov the vanity. This light is controlled by a switch when you walk into the bathroom. I want to be sure the fan is being used when the bathroom is in use (rental property where the if the tenants had the option they would choose not to run the fan to save money), therefore I would like only ONE switch in the bathroom, so the vent/light will go on when the light above the vanity goes on.
Is it possible to do this? And how would it be wired?