Expansion tank "pop"

[audiophool]

The other day out of the blue, wife heard a "pop" and the X5 3.0 dumped coolant.

Expansion tank split - classic, right? The one in the car was Meyle and I've ordered a new Behr tank and the ATF thermostat as it was broken when the tank came off. Strange thing though, there was a chunk of plastic sticking out of the split line. I cut the tank apart to look inside (I'm curious), and that chunk of plastic came out of the well where the ATF thermostat sits. The internal ribs of the tank are also broken. So, which came first?

Also, when I removed the tank, I found there was no electrical connection to the level sensor and I can't see the wire. Anybody know where on the harness the sensor wire comes out?