Morning Colt, One of the failure modes of the silver Control Unit, the brain box that runs the Webasto burner is failure to shut down after the end of the cool down cycle. Purchase a new one to solve the problem.
If you have a 12 vdc burner, red writing on the control unit, I have a rebuilt control unit for $500. Or I have a new Aqua Hot designed unit that is all solid state for a mere $990. I can ship today and will pay ground shipping from the Houston, TX, area.
7 one 3 8 one 8 3234
There are other sources on the internet. Do what is best for you