When I got a Thermo top C (BMW version) heater from eBay it took me a few weeks to get it working right. Mostly because of lack of and wrong information, also mine had a faulty transistor.

The fault.

When I first plugged in my thermo top it used a lot of power instantly, I worked out that the heater plug was on all of the time. It was a faulty smd transistor that drives the mosfet for the heater plug. If on one of these heaters, the fan, pump or heater plug runs with out any on signal being sent to the data pins then there may be a faulty component. They are not normally difficult to fix as long as you can do some smd soldering.

Heater configuration and start up.

Online a lot of people say just applying power to pin 1 or 2 will start the heater but this is not always the case. With the BMW version and maybe some others it wont start without serial communication from the car but this can be changed with the engineers version of Webasto's test software. As normal the people on forums and YouTube who have that software will not share it or want a lot of money for it. I eventually found it and uploaded it to the Internet archive so anyone can get their heater working. I will make another guide on how to save your heaters configuration and upload a new one. Note that the heater also won't startup if the fan, heater plug, fuel pump or water pump is missing.

Thermo top C Pin out.

Fuel pump type.

Reading online and some sellers show that the pump is the standard 22ml pump that comes with the Chinese diesel heaters, however this is wrong. Webasto's data sheet shows that the pump should be the 63ml type. On eBay 63ml is really expensive but the 65ml ones are cheap so just get one of those.

Other problems to look out for.

If your heater keeps going out and/or starts blowing a lot of smoke. It could be erther the wrong fuel pump, burning chamber needing a clean or the burning chamber needs a new atomiser mesh.