They are very easy to replace:
- Make a note of where the two ends meet on the old one, then just grab hold of an end and start to pull it off. Some bits are tucked behind trim but unless you are extremely violent, nothing should hurt.
- Starting from the same place as the old join, push/tap the new seal into place, and then start working around the door frame tapping it on as you go. I start with the bottom.
- Once you are about half way around, start with the other end, butting it up to your first piece, and then work backwards. This just means if the seal is slightly long or short you can lose it in the corners rather than faffing about trying to get the ends to match nicely when you finish.
Only place I could get one from was my local VW dealer. It wasn't cheap, but then what is... It made me chuckle when I turned up to collect it expecting a small roll of trim rubber, and it was handed to me attached to a piece of card the size of a van door. I think it is to stop it getting misshapen and twisted.
I had a really bad driver's side one, so got a new one there, and then the passenger side wasn't as bad, but still not great. Instead of buying a second new one I cut a decent section out of the old driver's side (they only rust at the bottom) and then used that to replace the bad bit on the passenger side. So yes I have two joins in the one on the passenger side instead of one, but you would have to be pretty sharp eyed to spot that, and it saved throwing another entire one away.
Just avoid having any join along the bottom edge and make sure they are some way up the sides.