Coolant and Heating Testing radiator fan
The connector to the radiator temperature sensor is on the lower right side of the radiator looking from the front of the van Remove lower grill and try not to loose the expanding plastic clips, you can use a large adjustable spanner to unscrew the sensor if you are going to replace it, have the new one waiting to screw in and you wont lose that much coolant.
Late 3 pin sensor about 83 onwards The electric connector pulls off and has three female spade connectors, jump the middle pin to each of the outside pins in turn (use a big paper clip), one side should bring the high fan on and the other should bring the low fan on. If one side or the other doesn't come on there is likely a fault with the fan or the wiring to earth. If the fan does come on with this test but it isn't when driving and the temperature gauge is getting past half way in traffic with no fan sound it is probably the sensor/switch that needs replacing 2012 cost £13 Brickwerks http://www.brickwerks.co.uk/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=527&category_id=120&vmcchk=1&option=com_virtuemart&Itemid=3
red blue wire is the high speed fan side, red black is the low speed. The centre is red white and comes from the fuse, so this is switched on the positive side. A good place to start is check the earth points above the fuse box with the crowns of earths. http://wiki.club8090.co.uk/index.php/VW_Electrics_bad_earths Also your fan motor may have burnt out or it may be catching the plastic shroud preventing it from turning. drop the spare wheel to get in there and take a look, mind them fingers in the fan though!