Maybe HT will be nice enough to post a wiring diagram for the tail lights for you and if dash & ashtray lights aren't on the same fuse that diagram as well....If they are on different fuses, that will help, look at both them and see what circuits they share....
To save on fuses, what you can do is get a 12v test light, hook the clip to a 12v power source (batt + if it will reach), take out the fuse and probe the load side of the circuit (the one that doesn't have 12v with the fuse out) if the light turns on that would indicate a short in the circuit, You can start disconnecting bulbs, connecters, isolating circuits until the light goes out. Once it goes out, you've taken the short out of the circuit...