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All the diagrams you could possibly want for the Series 3 are available, stickied on the top of this list under "XJ6 & XJ12 'HOW TO' quick links" then choose the "Wiring Diagrams S57 Ser111 XJ6 (kindly supplied by Doug)" link.
Here's just the pertinent bit you're looking for...
Basically check for power on the motor when activating the switch. If you have it then the motor is the culprit, if not then look for ground on 83Z and power on 83Y on the switch. If you're missing ground follow the Black wire to the chassis ground. If you're missing power check the in-line fuse that's behind the carpeting on the center hump by the seat.