It wont usually be the motor that has gone and you can test it by putting 12v to it, if the gears do not solve the issue then it is likely to be the wear on the lid/casing as covered at the end of my how to.
The solution is to have a small shim made to sleeve over the worm gear where it sits in the casing which does work as I tried it on one of mine, if the casing is worn and without the shim the new worm gear sits out of true in the slot and will not mesh with the other gear. I had hoped to get some shims made at the time but the person who was going to make them for me sold his lathe.
There is some good news though, when I originally sourced the replacement gears from Turkey, there were no spares for these motors available, but now there are various choices on E Bay:
New motor ( expensive )
https://www.ebay.co.uk/itm/Exterior-...4AAOSwA89d~H6J
Replacement gears:
https://www.ebay.co.uk/itm/Rover-75-...UAAOSwFNhgMVBi
Casings:
https://www.ebay.co.uk/itm/Mirror-Mo...gAAOSwE6ReVh3w
New Lid:
https://www.ebay.co.uk/itm/LAND-ROVE...kAAOSwTuJYnIQu
I have a spare motor or 2 which are not working and have just ordered a casing and a lid and will report back when I have them fitted.
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Coven near Wolverhampton WV9
2000 Cowley Built Rover 75 2.0 V6 Club SE/ Atlantic Blue/ Sandstone/Walnut (daily driver)
2003 Land Rover Freelander 1 TD4 HSE Helsinki Blue/Black Leather (swmbo daily driver)