Estate letting in water
 

My 1996 classic estate lets in water around the tail gate somewhere.
Sat in the boot with a torch after giving the car a hose and it seems to drip from underneath the plastic trim where there is an access panel to get to the rear light bulbs. Can't for the life of me see how its getting in - all seals are in good nick. It's not mega serious as the water just runs in to the spare wheel well and sloshes around until running out the drain holes I drilled - just a bit annoying and I'd like to sort it. Anyone come across it before?
TIA

Try removing the rear light units, put some new sealer around and replace them.

Thanks but that was the first thing I tried - thing is it runs down the inside of the plastic trim and then drips out of the access hatch so it must be getting in above there and away from the lights.