I noticed some before I left for France, and had tried to remove it with little bags of silica gel suspended inside the bulb holes, with the bulbs in place, but to no avail.

Yesterday, having half an hour to spare, I removed all the bulbs, and poked bits of rolled up paper towels into the light unit.

First off, complete towels, rolled into length along the long diagonal. They came out soaking

After FOUR were completely wet, I couldn't seem to get any more, so I resported to: quarters of a sheet, rooled up and held on the end of a length of semi-felxible wire.

Furkling them about inside the light, while watching from outside, I could SEE the water being soaked up along the bottom front edge Again, it took a good THREE towels to clear it out.

I left it open, facing the afternoon French sun, and am hoping for no more. If it comes back, I'll have to remove the light and try and find the ingress

 

Hi alcazar,
Have noticed this is a bit of a problem with classic shape cars. Mine has extensive black tape between the boot seal and the edge of the light (only visible when the boot is up). The gutter arrangement funnels all of the water that falls on the rear section of the roof, rear screen and boot lid down either side of the boot and over this area.
Maybe try some tape and see if it cures the problem! My car has quite a lot of black electrical tape that prevents water getting where it shouldn't!