One problem related to manifold/header/muffler/tail pipe has been reported for the 1999 Mercury Mystique. The most recently reported issues are listed below.
I was driving along when I could smell my car getting hot, so I pulled over and sure enough it was overheating, but did not show inside on the dashboard, so I had it checked and the plastic housing around the thermostat was cracked in 3 places, so I say fix it ,they said it would be 450. 00, ok no problem, then they called back and said that the housing was welded to the manifold and that I would have to replace all for 1200. 00 and that the housing wasn't made of plastic anymore because of this problem. I think this should be a recall and fixed for free.