Make new seals if the y are to thin stick one on top of other one. Also stop light in center of boot could have the same problem.use same procedure. Don't silicone seal to the body,you need to be able to change a bulb.
This video should be higher up the suggestions on search results to fix this problem as it's great advice - I spent ages trying to stop leaks in the boot and ended up breaking one of the lights on a Toyota Aygo (which resulted in ear ache as it's my Girlfriend's car). Took your advice and got 2 new sponges and made one myself for the high brake light, also bought a new boot seal as that had worn out. Leaving the old sponges on after siliconing just resulted in condensation in the lights as they didn't repel the water. For all the messing about on a weekend trying to sort it I'd have been better off just buying / making new seals. I wish I'd seen your video first, many thanks
