The fan clutch does not engage until 100-105`c and it is controled from two sources. The first is the temp switch in the head(left front next to valve cover, has 3 pins)and the second is the temp switch in the a/c drier. The most common problems are 1)the fuse, I know you said you checked them but just go ahead and replace them to be sure 2) the temp switch on the cylinder head.
You can test the system from the temp. sensor on the head by unplugging the two pin plug on the temp sensor and jumping across them with a piece of wire, the fan clutch should engage (so watch you fingers!). The temp. sensor in the drier is the one with the "pig tail" on it, if you jumper a wire across the two wires the aux. fans and the engine fan should engage.
hope this helps,
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop