i recently replaced the handle on my 1995 530. the frame assembly of the door handle that mounts on the inside of the door had cracked from fatigue (think of how many times that door's been opened on your '91). since the mechanism itself that opens the door is pretty basic (it's a lever w/ a ball bearing roller on the end of the lever, kinda), i would start there. if you remove the door panel you can do it yourself. to get the plastic cover off the door lever (the cover on the outside of the door), open your door and right above the door lock of the door (not the I-pillar of the frame) there should be a small plastic cap about an inch in length near the door handle, pry it out, take a flashlight and push on the metal lever that holds the plastic cap/cover onto the door. you can take the whole lock assembly out, do it with the window closed, and get ready to scrape a few knuckles doing it. a dealer or authorized shop will have to replace the lock itself since it's got something to do with theft prevention. the door lock assy is around $175 US. good luck.