Just a shot in the dark, but take the rear seat bench out and take a close look at the battery compartment. The PSE pump and rear fuse box live in there too. In addition to central locking, the PSE pump is also responsible for the Anti-Theft Alarm. If there's any water in there it may cause the problem you're having. Soggy PSE pumps are fairly common on SLK's, but I've seen a few 210's with the same problem. Let us know what you find.