I did this to one of my light clusters at the weekend as well. I took off the cluster (and drained all the water out of it!), cleaned off all the old sealant from both the car body and the rear of the cluster then applied 8mm Butyl sealant cord to the rear of the light cluster and re-attached it to the car. It was pretty simple. The sealant cord was easy to apply. There is a pretty obvious channel in the back of the light cluster where it looks like it should run. Just need to do the other one.
I used this sealant.
http://www.waysideadhesives.com/Masters ... 3_prod.phpNot sure if water was coming via the individual light lenses but it was clear water was coming in through the cluster sealant. When I took the cluster out you could see the sealant was shot with gaps in various places. It also looked like strips of blue-tac on mine. Probably a bodge by a previous owner.