The door locks have a hole in the bottom side that allows water to drip out (when water gets into the lock). If you exchange the locks from left to right and right to left, you end up with locks that have the hole on the top...water can get in, but cannot get out. This swapping solution, therefore, will only help for a couple of months (before both locks will suffer a problem).
You can take the lock apart and clean the inner parts (make sure that you do not mess the alignment up). After putting the clean parts back in the cleaned lock, you probably will see one or two parts that just stick out the cylinder; file these parts till they do not stick out any more and all will function well again.
Replacement locks cost about $50,- new in the netherlands. We can order new locks by chassis# for about $50,-!!! The new lock will fit the original car key.
with a minor 600+k