In *most* cases, the unit ceases to function because the works get all gummed up with old oils and greases. The unit can be taken apart and cleaned easily enough, but I should caution you if you decide to do so, as there are several springs, and other little bits that are dying to catapult themselves around your shop.
I've got some pictures of one being disassembled/assembled, but if you absolutely don't want to work on it yourself, I can have a go at it. Unless some of the vital components are damaged beyond use, it'll still be a fair sight less expensive than an entire control unit.
Pictures starting on this album page:
https://www.spcarsplus.com/gallery3/index.php/reMKIII/electrical_work
Cleaned and ready to *sparingly* lubricate and assemble: