why does the fuse to the night back lights trip?

Check the lightbulb sockets and make sure there is no corrosion in there for starters. The bulbs need a good ground and rust is not good. Sandpaper it out.

that is a tough question. When you start to deal with automotive electrical it can get tricky. Sounds like a wire is either pinched somewhere or the plastic covering on the wire has rubbed off and now it is making contact with the body of the car and shorting out. An electrical specialist would be able to make a better diagnosis of this than I could over the computer. One thing to do would be to take the tail light bulbs out and use a multimeter to check for a short to the body of the car. That is where I would start. It may be a problem near the tail-lights (sometimes a wire can get pinched during assembly of the car) or somewhere in the dash. Also, make sure you are using the right amp fuse (should be a 5 or 10). Sorry I can't be of more help over the computer, but there are a lot of variables to check.

There is a short in the wiring most likely. The insulation on the wire may have gotten either worn or cut through to the copper wire. Take the car in to a repair shop, and have them troubleshoot the problem. Good luck.

short in wire,switch or light itself...

you need an auto electrician