I have a new to me 2003 ML500 and my radiator fan is not coming on. We have replace the engine cooling temp sensor, checked all fuses and relays that would be associated with this and also checked the thermostat. Any idea on other things that we should check or what might be the problem?

ML 55's and 500's have a special fuse off the battery to run the fans.

The fuse is good.

Then you have to check the wiring at the fan itself.

Check the RED wire for 12v and check the BROWN wire for ground.

The WHITE wire is the signal wire from the ECM to turn the fan on.

Grab a volt meter and see if its getting any power in the line at all

Where can I buy one of these fuses, item 21 on your diagram

If you can't find it, option 2.

Ignore this post and see the one below.

At a M/B dealership and the correct one depends on the last six numbers of your vin#.