I have a detail picture of the up ended top section of the toilet [the works bit ] from my caravan book of the old style Thetford which yours maybe. It shows the position of the switch but by the look of it a bugger to get at in situ, he may have to seperate the works bit from the rest of the casing but how that's done they very conveniently don't tell you that. Why do the make these things so hard to get at. He might be able to have a look with aid of a mirror after removing the waste holding tank and check all is attached.
tip with micro switches dampness can cause malfunction try what I did and played a hairdryer on a tap switch that was playing up
It was fine after about 15mins med heat.
if you want the picture email email@example.com
and I'll send it.
Another tip given by caravan service engineer never completly remove the pump cable if renewing pump. You can't get the new one in. He said just cut the existing cable and splice the new to it [Terminal block] tape with waterproof tape and stick the joined cable up on the shelf inside at the back of the water tank. That's what I've done.