Hi Hammer

It is the metal inside !(aluminium i think)i have changed them on three doors.
It's easy to change, just pull up in the seal strip, you may have to use a screwdriver as a leaver to lift it up at first (remember to protect the paint!)on the front doors you have to loosen the mirror to gain access.
You just press down the new seal strip...That's it !
Price: about €25-30 each, as far as i remember.

There was no corrosion underneath mine.

Greetings

Christian U
155 2,5 V6