use a multi meter or on vehicle screwdrive type tester for low voltage. you can get one with a cable piercing spike, i use one of these just ensure you don't spike both conductors or you'll get short out maybe.
follow this: you may find a solution in the pdf's of your model if you pick the correct one.
uk & europe www.thetford-europe.com
australia www.thetford.com.au (even get request from australian campers)
australia site has stockist list.
home page select english {if in australia forget this selection it doesn't come up)
select customer service (top line)
select products from top line
select bulit in or portable which ever applies.
select user information from list on right.
select repair instructions from list on right
select built in or portable which ever applies.
select select your toilet series model number from right list.
you will now be in the pdf listings for your range of toilets just select those that apply to your toilet and down load them they're free.
as you go through watch for 'ask thetford' on left listings
you may not find what you want and you can send them a message.
for safety reasons which i agree with thetford do not offer repair advice for fridges. refer to service centre
download save to disc and use on hols if you take laptop or dvd player that way you have instant solutions.
Bob, March 2011