That's fairly standard condition for many of the rear inner window trims on our cars now. Thankfully, corrosion generally seems to be contained to the trim itself and a couple of the steel clips holding it to the door - I believe new steel clips for the trim are available from Rimmers (should you find a decent replacement trim). The trim and the clips pull up from the door - shouldn't be too difficult, but care and a pair of long-nose pliers will help.
Too late to be telling you now
but when one releases all the outer clips from a door card, the next step is to simultaneously lift the door card UP and out. This will let you remove the door card without taking the riveted steel clip away from the door itself. The plastic clip on the door card can be removed by sliding it down (or up) from the steel plate it's attached to on the rear of the door card. Long-nose pliers again will let you depress the two sides of the steel riveted clip to facilitate removal of the plastic clip therefrom. Once separated, you need to get the steel clip re-riveted to the door and the plastic clip refitted to the steel plate (on door card) before refitting the door card.
Red clips are not for the door card. Get a handful of the big white clips, a few of the grey clips and one white fir tree (multi-layered) clip - this one fits at the top rear-most area of the door card.
I'll see if I can get some pics to help..
Cheers,
Karl.