Thetford Toilet Reed Switch

Had a service done recently at the house and had a faulty reed switch guessing it is always showing red insead of green if ok (this is a guess as we have just bought the van) anyway engineer didn't have one and was going to buy one from Ebay and do myself. Question is it a straight forward job and does anyone have a link to a "how to change over" guide and possibly some pictures of the job, many thanks in advance, Stephen,