My 89 is doing the same thing. It came on over time with a gradual deterioration. I'm sure it's just a mechanical linkage component loosening up as all of the vacuum actuators work fine (I generally unlock from the trunk). I'm taking mine in for a 60K soon and am going to ask them (independent shop) what they'll charge to fix. Depending, I might wait till spring (it's been snowing here) and fix myself, or if not too unreasonable, have them do it. I'll let you know what estimate is. Probably about a week or so.