I just bought a 1987 Samurai tin top and according to the previous owner the rear defroster does not work but it is getting power. Is there anything i can do to fix it without replacing the entire window?

short answer is 'no'

have you put a multi-meter on it to see where the open is?

If it is between a strip and end bar you may be able to repair; if the strips/elements are compromised, you may need a new window glass....

you might also try this once you find thew break:
http://www.amazon.com/Permatex-15067-Window-Defogger-Repair/dp/B000HBI9YQ/ref=sr_1_1?ie=UTF8&qid=1340744964&sr=8-1&keywords=rear+defogger+repair

That permatex stuff does a pretty good job.

Finding the breaks can be challenging - get a DMM set it for continuity and clip one lead onto the grid terminal on one side of the glass - wrap the other lead in a 1" x 2" aluminum foil flag and then rub the foil down the traces - watch the meter for ANY sudden change in reading.

If the trace is intact you'll see a gradual increase as you move toward the center of the screen followed by a gradual decrease once you go past the center, but if the trace is broken, you'll see a sudden change when you cross the break, and then a close visual examination will show the actual problem.