seat belts not retracting...fixable?

Mine and my friend's car stang does the same thing, but they go up a decent amout once you open the door, does your do that? There's some kinda retractor controlled by a black plastic door switch if you look near the inside of the outer door frame somewhere I think that's it. Our belts tend to loosen up on their own tho, that sucks I gotta keep yanking it out to make it retight.