I would have the body shop fix it. The central locking system with its vacuum and electrical connections is complicated to diagnose. It could be a loose vacuum fitting somewhere, probably where they replaced or a bad loose connection somewhere. It could be something else in the system going going bad.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T