I have this little board for just this reason: http://www.pinitech.com/products/ballystern_cabswitch_tester.php
He's out of stock but he may have it in a kit.
Make sure the J3 connector (specifically pin 1) on the MPU and the J2 (pin 7) connector on the SDB look good. If they do make sure they have continuity from the switch to the boards