I have a 1989 SE, and it seems this is a fairly common problem. It drove me nuts for the past year to have to use my trip odometer to fill the tank based on mileage
Here is a fix for the problem - assuming you can do some light mechanical work and you know how to solder. Its really easy, and make sure if you do this to include the heat shrink tubing to cover the transistor leads so they don't short. Also make sure you have thermal paste on the base of the transistor
http://www.nissanpathfinders.com/Solution2.pdf
The voltage regulator (that apparently costs $60) can be replaced. I got an NTE964 instead of the KA7808 - but its interchangable - same pinout and readings, etc. and the thing cost me $2.25
I just (quite literally) got done doing this and it fixed the problem
Good luck