One option is to retrofit a red top aux fan switch with a 100*C setpoint from a 102 or 601 engine. Here is a 2-pole red top 100*C switch from a 102 engine -

http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1986@@1986&makeid=63@@MERCEDES+BENZ@@X&modelid=6127:ED|10000009:SM|846:MBC|1484@@190E+2.3&catid=240909@@Cooling+System&subcatid=241005@@Auxiliary+Fan+Switch&applicationid=W0133-1630156&mode=PA

Here is a gray top 3-pole 105*C/120*C switch from an early 602 engine -

http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1986@@1986&makeid=63@@MERCEDES+BENZ@@X&modelid=6127:ED|10000009:SM|846:MBC|1484@@190E+2.3&catid=240909@@Cooling+System&subcatid=241005@@Auxiliary+Fan+Switch&applicationid=W0133-1625972&mode=PA

Fastlane doesn't list the red top 3-pole 100*C/105*C switch I pinched from a 601 at the yard.

You must have been raised in a cave if you're not familiar with -

http://www.youtube.com/watch?v=IMxWLuOFyZM#t=2m0s

Sixto
87 300D