the fan is negatively switched, there is a relay that is used to switch it on.
first thing to do is check that a live signal is getting to the fan.
if it isnt check the fuse and connections at the fuse and fan.
next check the relay's connections are clean and do a continutiy test from fan to relay and relay to earth. if it all tests ok you know the problem is not the fan circuit, the problem must be with something that sends the switching signal to the fan relay (many possibilties )