Pentoman is right about the switch test. Lift the connector a bit but do not remove it. With car running shorten the pins with a screwdriver. You should hear a click as the fan cluth does engage and the speed of it will increase. You can't really miss it because it start to blow a lot of hot air at you. This is on 190 E with 2 pins
On the 190 Diesels the switch has only got one pin and you can test this switch by pulling of the whole connector and insert a small screwdriver into it. Then you ground screwdriver at for example on the valve cover and voila...cluth does engage. Otherwise there can be wire or cluth problems.
I did this a week ago so I haven't mixed things up yet...
Also...you stated you changed thermostat. Do you know at what temp the thermostat do open up? I have seen at least two thermos for 190E. One that does open at 79 degrees and the other 85 degrees (both celsius, I'm european
If you live in a "warm" area you should get the 79. Don't know if this is recommendable but you can boil the thermostat to see at what temp it do open...Measure at the same time and you got it. If you don't know which one you got...
And I'm not really a mechanic...just trying to help.