Could be a leak or disconnected drain hose on the antenna, or you might have a rust-through problem around the wheel wells, but the most likely source is the taillight gaskets. The three studs that you pull in to seal each taillight gasket are set in the plastic of the lens. On my '95, all six of these were broken from someone turning the nuts way too tight. I was able to reset the studs into the lenses with Duro plastic welder epoxy, and got a seal by resetting the gasket on the lens after cleaning both and adding a strip of 3M strip calk to the gasket surface that fits against the metal opening.
Good luck. I know this can be frustrating.