You still have the old motor???
That had a 3 wire switch for the high fan ..think it was on the head.
Anyway, according to that schematic [ pre-88]..the fuse is "D" and the relay is K9
That wire to the sensor is br/gy..look for that wire . if you ground it, the fan will come ON if they left the relay and fuse intact.
try it and let me know
if that is the case , all you have to do is put a sensor in there that grounds when temp is high [ the benz one for 190 is 100c, the E is 105...] and hook that wire to it....or bring that wire to a cabin sw and ground the other side of the sw.
Here is what was on the old engine
http://catalog.peachparts.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleAAA&cookieID=2EI1B4MYE2FD0ZMMN4&yearid=1986%40%401986&makeid=MB%40%40MB%40%40X&modelid=300%2DE%2D002%40%40300E&catid=G%40%40Cooling+System&mode=PA&subcatid=G5045@@Engine+Temp%2E+Sensor&source=www.peachparts.com&clientid=catalog.mercedesshop
..but if you do not have that one, I would use the 190 one instead..b/c it is 100C and that would help the a/c as you will only have the single speed now.. The RED one:
http://catalog.peachparts.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleAAA&cookieID=2EI1B4MYE2FD0ZYXGJ&yearid=1987%40%401987&makeid=MB%40%40MB%40%40X&modelid=190%2DE%2D003%40%40190E++2%2E3+16%2DValve&catid=G%40%40Cooling+System&mode=PA&subcatid=G5030@@Auxiliary+Fan+Switch&source=www.peachparts.com&clientid=catalog.mercedesshop