I went and put the original housing and t-stat back in but used the o-ring off the new, obviously wrong housing-t-stat combo. So far no leaks. Looks like I bought a $40 o-ring. I cant understand how that BMW dude could be so insanely incompetent. At the same time, I blame myself. I just figured those guys new what they were doing. Plus he was kinda older with a beard and I trusted him. I'll never make that mistake again!