Would suspect a problemattic relay or somesuch, or possibly a bad earth. If you are getting voltage but it has nowhere to go then the fan will not work (I'm sure you know this so why am I telling you ) check if there's voltage on the earth side of the fan with it connected (if possible) that will tell you if it's a dodgy earth I think.
Or are they negatively switched? Hmmm could be a problem.
with the 155 it's located next to the steering wheel and the centre colom, at knee height. But then it would require the aircon to be running, imo. The point of turning of the aircon is to override it's commands, afterall.