The problem you describe could be related to a worn ignition switch. Try moving the ignition back to the left a little next time you start it with the fan switch on high and see if it comes on. If so, replace the ignition switch and your problem will be fixed.
Not a sure thing, but I have seen enough about this being the problem, it would be worth a try before you spend any more on parts or diagnostics.

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89 190E
2.6L
Arctic white
Grey leather
Sunroof
Pirelli P400 rubber
Smoked corners