Leaky rear lights tintop

Firstly Happy New year to you all. My mk1 has a few water leaks, in the front,  rust either in the the scuttle or around the windscreen, perhaps both, sunroof when it's heavy rain and in the boot through a small rust hole in tailgate under windscreen seal, the old girl needs a date with a welding torch, nothing too hideous but it's rust all the same. Anyway, I'll start with the easiest to attempt to fix, (not least because I can't weld) but apparently not to permanently solve..

I've had a few goes at stopping my tail lights from leaking over the years but it just keeps coming back after a while. I've cleaned it up, used hella over the frankani light units as well as SA copy smoked lights (don't bother they are rubbish) and also sticky back foam stuck to the rear panel which the seal on the light beds onto. The foam does work but seems to only be for so long before they start to leak again.

I'm sure I've read about using gutter sealant, I've also read clear silicone but I'm reluctant to silicone them to the car, as they'll be a nightmare to get back off, not to mention its a very crass thing to do in my opinion.

What methods have others used to stop them from leaking?

On my bunny's tail lights I just used a piece of Black Duct tape one inch above the lens to the rear valance to the top of the lens all the way across the to 3/4 of the way down on both sides.  81 Diesel Rabbit (USA) did this in 1990 sold the car in 2001 still had the duct tape on the thing never leaked.

Briano1234 duct tape? I'm not sure I 100% follow, you taped onto rear panel and the seal on the light unit sat on the duct tape? A bit like I've done with the foam I mentioned?

Tbh I think the design is a bit naff, the seal is only just proud of the plastic casting where the lugs are on the light unit, any slight discrepancy and the seal won't be doing much, more so being 40 year old seals. That being said, I'm unsure how exactly the water gets in, I'm assuming the lights themselves are watertight and the seal is the problem.

I think the seals are just foam/neoprene gaskets right. How about just adding a second layer of that to pack out the space between the light and the fixing point on the car so the seals are compressed properly no?

Shubbs some sort of foamy material, yes. Not sure you can buy replacement gaskets to be able to double them up. I had thought a layer of the foam I have underneath the original gasket but IIRC the gasket is stuck into the unit so didn't want to try pull it off and tear it, hence I stuck it to the rear panel instead. Either that or I was too lazy/short of time to put the foam under the original gasket.