1993 Ford Aerostar

You can see water going through the exhaust? You might remove the exhaust headers, see if you can tell which head.

As far as the rear lites, I'm not sure if your talking tail-lites or brake-lites or what?

As far as the water problem, I was thinking about a bad gasket allowing water to get in your exhaust system.

Usually it is the headlite switch that activates the dash and tail-lamps. I'm not saying the switch is faulty, but if you looked at the wiring diagrams in your manual, if it has diagrams for the tail-lamps, you could track the circuit from that switch or backtrack from the tail-lamps. I don't have a wiring diagram for your tail-lamps. You might also look at the tail-lite bulbs and sockets. I have found some problems by wiggling the electrical connectors.