I had the power lock on my passenger door die a year ago on my 89 XLT It turns out that it just started to get a little gummed up because it was old. The problem started when the winter hit and the low temps made the grease really tacky, by summer it didnt work anymore so i simply replaced the motor and its been fine ever since. Now the drivers side lock is begining to get gummed up itself and i will probably have to clean it out or replace that one too by the spring. But in response to Hawk's question, both locks should be power, at least in an 89XLT.
--Chris