Condensation in rear light?

Could be that you have water ingress in to the boot area, and the area inside the light unit where the bulb resides, is hot then cold etc,and with no air circulation, so I assume it attracts condensation, Fitzy

I think the water is coming in through the rear light.

How do you remote it?

These are just my suggestions as to how I would deal with the removal of water from the lamp units,,,,,,,

Remove the lamp unit and take indoors, or warm the outside of lamp with hair dryer while dabbing and wiping a cotton ball round the inside,

I would look for the cause of the problem to prevent this happening in the future, Fitzy

This was the problem, I couldn't see how to remove the lamp!

Now sorted.