As Ron says, a manual will help. I am not familiar with the Webasto details, but if it uses a snap type limit switch, you don't need to be afraid of buying a new one. They are very, very cheap. Even if it uses a more expensive home furnace type of limit switch, they aren't all that expensive. The snap switches are very prone to failure as they age.
Easy way to check is just to bypass the limit switch, once you find it, and operate the heater. See if it then reaches its proper temp and cycles normally on its thermostat. Obviously, you must stay near the unit at all times while running this test, since if the unit should really happen to be overheating, there is nothing to protect it while the switch is bypassed.
The cutout temp for either switch should be in the manual and usually will be marked on the switch. You can test it electrically for continuity below and above its cutoff point, or you can also test it by heating it (after removing it from the unit) in a pan of water using a thermometer to increase temp to the cutoff point. The snap switches snap audibly when they open up. Obviously, you can't immerse many of the furnace-type switches in water unless they have the long probe off the side, in which case you can.
Last edited by KeyPineSavage; 03-06-2009 at 10:18 PM.