I had the same problem with my 1990 190E 2.6 and indeed the problem was the thermostat. The thermostat would open just a crack and would be ok on the highway but in traffic the tempature shot up quick, even here in Chicago. The thermostat should be about $25 and takes about 3 minutes to replace. Three bolts and a rubber O-ring. You can remove the thermostat and check it very easily.
I would also check the electric cooling fans to make sure they are functioning. The should be coming on when the tempature goes up, if not, the most likely cause would be the relay. Just to mention one of my fans was also wired improperly and was blowing in the opposite direction of the other. Re-wiring the harness corrected the problem.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T