The panel that holds the HVAC control assembly in the dash came off in two pieces, oddly enough. Darn previous owners...
A longtime ago, I picked up one of the cheap Harbor Freight plastic welders just for this sort of occasion. Once again, that $50 investment has saved my butt.
I'll be using it a few more times before I'm done assembling the interior, as a number of panels have sustained damage from a clumsy previous owner or two. This has me wondering who else has dealt with broken plastic bits on their Mercedes.