There are several threads about this issue, but I thought I'd post my fix here... no drilling required!
The inside of one of my rear clusters was like a shower screen, and some drops had run down the inside to pool in the void between the lenses and the outer cover. It looked fugly.
I removed the unit (as above). There is a rubber/foam gasket between the cluster and the bodywork - this had somehow got kinked along the top edge (or just fitted badly) so there was a clear gap for moisture ingress. Judging by the crud around the gap, it had been like this from new. This may not be the case for everyone's lights which have condensation, but it may be worth a look.
I detached the bulbs and the damp and smelly wiring loom. Giving the unit a really good shake got rid of most of the pooled condensation, which got me thinking that it was a bit odd that water could escape from a sealed cluster
Now here's the thing... The method which worked for me to drive out the residual moisture was using the hose of a vacuum cleaner. With the power down low-ish, and placing the nozzle against the indicator bulb hole (at the larger end of the cluster) the misting in the void cleared very quickly.
So the unit is not sealed air tight - you could feel a lot of (dry) air being drawn through the other bulb holes.
After leaving the cluster in the sun for a bit and repeating with the vacuum, there is now no condensation at all. Gasket replaced properly, looks as good as new