Probably not a ground wire problem. Visually inspect the bulb sockets, making sure the correct bulbs are installed. Could be a short circuit inside the light switch. Somewhere, the 12v supply is going straight to ground similar to a penny in the cigarette socket.
I'll start by asking you a question. Has anyone either installed a new radio or messed with the wiring to the radio? The reason I ask is, the most common thing I've found when I see these symptoms is someone has used the "illumination" wire for the radio for a ground wire. If no one has been in that area you may need to inspect every light in that circuit for corrosion or anything shorted to ground. But I'd certainly check out the radio IF anything has been changed or disturbed in that area.