If you can't find it, you could wire in a new one between the fuse panel and the pump, putting the switch in a new location so you don't have to fish wire so far. Also, have you checked that the switch is sending 12v out when switched on? A lighted switch will have 3 wires, (+) in, (-) in, and the switched 12 v (+) out. Sometimes water pumps are wired to have full time 12 (+) and the switch is on the ground, so test to see if that's the case.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI