it's most likely either a short in the wire, or a bad sending unit. if you pull off the accordian hose, and look under the powersteering resivour to the left about 4", you'll see a 2" round thing sticking out of the side of your block. it has a rubber cap on it. pull off the rubber cap, take the wire off, and clean the connections first. (all the while looking for corrosion/ shorts/ bare wire etc...)
after you've cleaned it up and determined it's not a short or a bad connection, then you might want to pick up a known good oil pressure sending unit, used is ok as long as it works.
-shaeff