I have a 1989 190E 2.6 and the fan would not work except on defrost also. I replaced module in dash that had all buttons and temperature wheel and did not fix problem. Then by accident I discovered that when I turned key to crank car, the key went about 1/8 inch too far. I carefully turned key backwards 1/8 inch with the motor still running and the fan worked perfectly. I put old module back in dash, cranked the car, turned key backwards, and the fan worked perfectly. This sounds crazy but its true. Now I have to make sure I turn key backwards so heat and fan will function properly.