I did this on the passenger side A pillar trim, removal is ‘quite’ easy, remove the sun visor and get a trim tool or flat screwdriver push it in the join in the same area where the hinge for the visor goes and lever it apart. Then you just get a trim tool or if desperate a flat screwdriver blade with tape on. Lift the rubber that runs down the outside of the pillar trim to expose a gap about six inches from the top, grab your trim tool, sod it you need something stronger, I used a small metal pry bar with tape over the end, be brave push the tool in about half way across the pillar trim. Lever the trim away from the pillar towards the centre of the car in a quick motion, it will be quite stiff!
Keep going down the pillar six inches at a time until the trim is free.
Reassembly is a reverse of removal, I will say though you have to be very patient with getting the bottom end in, make sure to get the three ‘tangs’ in their slots first keeping the trim as close to the pillar as you can reasonably get without fouling anything else. Once it’s in just get the rubber over it and you’re done. Just be patient getting the bottom end is is a b*tch