I don't know what it is about vehicle repair. Around here we only try and do it when its cold out, preferable with a gale force wind blowing snow at the parts to be replaced, banged, or sworn at. Not necessarily in that order.
Who would make a thermostat housing out of plastic? Some one who works for a company that believes consumers only consume, and therfore need to make parts so they will have a very limited lifespan..
Chick Weed, and they chowed down on that with grunts of delight!