unfortunately its trial and error, you are probably best speaking to a VW specialist, go for the cheapest option then work your way up, if you like i can give you the name of a guy who is excellent with SEAT's he may well have VW knowledge too, he was the one who diagnosed mine, dealers were no use whatsoever! the difference between my fault and yours is my car would start up then cut out after about a second and the immobiliser light would flash constantly