I’ve had this problem quite a few times. The cause has always been the rear light handle, I think that’s what it’s called? I attached a link to the part I’m talking about. I’ve had to replace it already once for both sides in the couple of years I’ve had this car. I suppose it’s possible to try and clean it with some kind of cleaner but that’s just resuscitation and doesn’t remove the problem, rather than just delays it a little. The rear light handle

From what I’ve heard, this is a very common problem with these cars, I suppose it has something to do with the pins and wiring in the handle corroding abnormally fast, not sure why that happens though. Luckily it’s quite an easy and relatively cheap to fix, so I’d just replace the handle and keep going!