On my '95 Aerostar the tail lights go off when the headlights are turned on! Anyone know the cure? Thanks

 

Can't say for sure if this is the cure.
The headlights consume a fairly large amount of current to operate. To have a complete circuit to operate , the headlights have to have a good ground . I would suspect that the ground for the headlights is non-functional and therefore it is using the circuit for the tail lights to get it's ground. The cure very possibly might be to provide a direct wire to the headlights for their ground or just go and look for,clean and tighten all the ground connections you can find that come from the negative battery terminal to the various sheet metal points. One of them is either faulty or disconnected. If you do this when the headlights are on , you will know which one it was as soon as the tail lights come back on. You will be surprised when you find it!!