I take no credit for this - its lifted straight from the Fiesta Owners Club (although I've truncated how to replace the speakers themselves. Its pretty obvious when you get the trim off. Did this myself to put in the Mondeo premium speaker upgrade.
Original guide here:
http://www.fiestaownersclub.com/viewtopic.php?f=22&t=27This post is a complete guide to removing the rear speakers in a 3-door Mk6 Fiesta and fitting aftermarket ones
There may be other ways to do this, but I believe this procedure to be quite efficient - don't be put off, it is actually very easy to do. The Mk6 Haynes Manual is useless and doesn't explain this at all.
Step 1: Remove the bottom seatbelt loop by unscrewing the Torx Bolt with an T50 Torx Bit. It helps to move the seat as far forward as possible to do this. Remember the order of the washers: Small Black Washer, Large Black Washer, Small Brass Washer, Cream Plastic Washer. Unhook the seat belt from the bar, then wiggle and pull it to unhook it from the plastic trim at the rear.
Step 2: Lift up the rear seat base and fold it forward on its hinge. You could remove the seat base if you want more space by undoing the screws on each hinge at the front of the seat, but I left it on. Fold down the rear seat quarter on the side you are working on and remove the seat back by undoing the four Torx bolts with a T40 bit.
Step 3: Where the rear plastic trim meets the door sill plastic trim lift and pull it up - it is clipped down. The two pieces are hooked together and need separating. Now pull away at the rear side trim starting at the bottom. There are clips in the locations shown in the photo - just pull to unclip. Beware of the one at the base of the seat hinge. This is a plastic fastener that needs pulling out - it is difficult to remove! When unclipped lift the rear panel clear.
Step 4: Remove the four screws holding the speaker in place. Keep safe for later. Unclip the speaker connector by pushing in the retaining latch.
Replace the speaker
Now refit the trim, ensuring all clips are pressed until they pop, and that the doorseal rubber is not trapped. Re-fit the rear seat back and fold the seat base back down. Re-install the seatbelt bar making sure you hook the belt on first.