My brother inlaws 80l waeco was doing the same thing, upon pulling it apart, i found waeco had already installed a second thermostat in antisapation that the first would fail? Was a matter of unplugging the old one and plugging in the new one wich was curled up and cable tied to some existing wiring. By the sounds of things I will have fun replacing the next (cutting the insulation) as there certanly isn't a third wrapped up there.
Lemo.