Hi Kev, and welcome to the board.
Now then this could be a couple of things....Firstly the rear lights. What I did to seal .the rear lights was NOT to silicone them into place as this causes problems if you have to change a bulb, later, rather than that I got some waterproof draught excluder (On a roll, you can get it from B&Q or Wilkinsons) and stick it around the hole on the back of the lights, where the foam seal was. You can also run it around the back, inside edges of the lights (about half an inch in, so it doesn't show) to seal the outer edges of the light cluster to the bodywork.
I had a problem with the back lights misting up and whilst I was doing that, I drilled two holes in the black, back of the light cluster, right at the extreme edges, got a hair dryer in there for about 5 minutes. Be careful when you are drilling but that is the only way, as the lens is sealed to the reflector but once it has dried, get some silicone sealant and put a smear of it all around the edge of the light between the black back and the lens. Then put a dab of silicone in each drill hole to seal the unit up again. I had problems with misty back lights for years but since I did this, last year, haven't had the problem since.
The other place that your leak could be coming from is the high level brake light. To sort that, unscrew it, take out the light cluster and other bits so that you just have the lens. Chances are that the seal has broken down so clear it all off from the back of the light and the bodywork and run a bead of silicone around it. Refit the unit and the silicone should "Squidge out" onto the bodywork but a damp paper towel will wipe it off....Don't leave the silicone to dry as it then becomes a Stanley Knife job and that's not good against paintwork.
It is a bit of a faff to get the rear light off but you'll need a 10mm spanner, there is only one nut but it's up behind the rear three-quarter panel trim and is a bit of a pig to get to but if you take your time, you can do all three lights, for little money, in a couple of hours.
All the best and I hope that helps.