I am posting this becase in three days searching and reading, here and the other F150 forums I could find nothing on removing the rear inside panels on a regular cab. So I used the brute force and ignorance method. So here is what I learned.
STX Regular Cab;
On the bottom of the tiny door is a single 6mm screw. Remove that. Push up a little on the panel, then pry the whole door panel off, getting a finger under an edge and pulling straight away from the door. It has 8 VERY firm clips holding it in place. There are two posts that stick out of the panel and into round holes in the door. these seem to be guide posts. some melted plastic, looked like ends on the guide posts, fell out of the panel when it came away. There are three hunks of insulation, sticky sided, that cover the three largest holes in the inner door skin. One of these fell off, but it's no problem to put it back.
The speaker is held in with four screws, and is identical to the ones in front. Part number XW7F-18808-AB, 25watts.
The wiring harness has an easy to unsnap hookup. The wire closest to the snap is the + wire on the speaker. I had to confirm that with a 1.5 volt battery. It's not marked anywhere. You can get an adaptor online or at Bestbuy or Circut City to make your new speakers fit. Metra part 72-5600 for the speaker harness adapter. These were 7 bucks per pair, cheap enough, but both stores locally were out of them... So 10 cents worth of solder, a nickles worth of shrink wrap and it's done.

Going to sell the four factory speakers and ends on E-bay, I guess.

The Pioneer speakers had a warning that they were too deep for extended cab F-150s.
Dunno, on the regular cab it's plenty deep. There is tons of room back there. You could cut the hole bigger and mount some monsters if you wanted...
I just used Pioneer TS-A6870R speakers in all four locations. Perfect fit.

On the fronts, you access the speakers by tapeing up the tip of a flatblade screwdriver, and inserting it in the slot in the front of the grill and prying up. Get it up just enough to grab with your fingers, then pop it off.
Note; On these TS-A6870R speakers you break off two little 'extra' mount tabs on the front speakers. Ten second job.

To put the back panel on, I started the eight clips, watching to make sure the two posts were in their holes, then sat on the floor to put in the one screw. On both sides I had to pull the panel down about a third of an inch before the hole lined up. After putting the bolt in, I had to go around the panel thumping it with my hand. 2-3 trips around before all the 8 clips worked their way in.
The front grills just snap back in place.
I was going to put in a small sub, but right now I'm more than happy with the base.

I'm not using an amp, just a Kenwood 622 mp3 player.
Chris