Why do you need a red light? You can tell when it is getting full by looking down the hole. Rip the cable off.
The electrics on cassette toilets are mostly totally unnecessary yet give constant problems. After replacing a couple of pumps and micro switches over the years I plumbed ours direct to the van pumped water system. All the problems have gone.
Cheers,
Peter
As Tony suggests, you can gain access to the mechanism if Tony's solution won't work. To get access, take off the slide blocking the inlet. There will be a rail or groove on each side where that slide slid into place. Find a piece of wood that fits snuggly between those slides. You will notice that the top has a circle, or removable top portion, on the side of this is an arrow (I think), it aligns up with a corresponding dot on the cassette body. Around the top on the body of the cassette is another dot. With the piece of timber fitted between the slides, turn the wood to rotate the top to the other dot. It lifts off and you have access. Assembly is in the reverse order.