From another post http://forum.club8090.co.uk/viewtopic.php?f=39&t=145802
bigherb wrote:It needs a relay for when it is on high speed it consumes more current than the switch is able to pass without burning out.
Which relay depends on which fan and relay you have. 300 watt with relay in the fuse box or 500w with the relay on the A pillar.