Well lets start with the basic fact that Chevrolet are completely unable to make any exterior light unit water tight
So what you need to do is add some drain holes at the bottom, the rear lens are quite easy to get out, two thumb screws under the carpet in the boot. To remove the offside R/H one you will need to loose the spare wheel and tip inwards into the trunk.
NB : Don't drop it into the spare wheel well otherwise it's a real pain in the ass !
Also tape a large cloth on each side of the bumper with insulation tape, because once th lens are out there is no where to rest, or each of the five bulbs you have to remove