www.imoc.co.uk

The inner arches into the boot are prone to rotting out, if driven in the wet water can get in. it's worth removing the carpet and looking up into the qtr voids if you haven't already done so.
Hopefully tho it's something basic like the boot seal or grommets.

Does a standard rear screen surround have an imprint on the back so it can be cut out to accept a 3rd brake light? Mine has recently been resprayed so don't really want to have to buy another surround with the lamp already fitted if I don't need to and it's not exactly a 2 min job to whip it off yo ...

The vents allow the displacement of air when you close the boot lid.

I'd be inclined to remove the seal and look for rot in the metal under it if it's leaking through the light, there's no other way water could get to that point.
I'm in the process of restoring my rev 3 tbar and the roof needed some attention with a welder.