Worn out weather stripping may also be to blame, there may not be enough tension on the lock. I'm in the middle of doing this project myself, might help? My seals are shot, flat as a pancake which lets in a lot of dust, not to mention the back door has not been staying fully shut, like when you don't close the door all the way, hope this fixes it by adding tension.
EDIT:fixed the back door problem, now stays shut by adding the tubing to the weather strip moulding....nice!
Ordered up a roll of this, actually was short and just ordered another