I just replaced my evaporator and heater cores, and if you frequent f150 message boards, you probably saw my posts asking how to get the freakin evap core out. Got a few pointers from some folks, and finally figured it out.
I'm not saying this is the proper, by the manual way to do it. But it did the trick for me. This may not remove the entire plenum, but it certainly made it loose enough to pull it away from the firewall, and I thought I could've pulled it all the way off if I wanted to.
First, if you haven't already taken the dash out, do a search for replacing your heater core, and there's an amazing write-up in a Word document on how to do it. Once you've got the dash out, hoses disconnected, and plenum cover removed, try this.
Most of these pics I took after I'd already disconnected them, so I apologize that it doesn't match exactly what you'll see.
Step 1: Remove this nut in between the holes in the firewall in the engine bay where the evaporator and heater cores connect. I think it's 10mm, could be 11mm. A flashlight helps to see it.
Step 2: Remove your battery completely, as this one is kinda tough to get to with it in there. Possible? Yes. But difficult. Standing at the passenger fender, looking at the battery, there is the same sort of screw as Step 1 located on the firewall in the engine bay, directly left of the negative terminal on the battery. There are actually TWO nuts on this screw. The top nut holds the accumulator brace and AC hose. Remove that and you'll discover a nut that looks identical to the one in Step 1.
Step 3: In the cab, remove the nut that's above the blower motor (passenger side), connected to the frame that goes across the entire dash. It's right above a vent at the very top of the entire plenum box.
Step 4: Underneath and to the left of the part of the plenum where the heater core is (right where the evaporator drains out, there's hidden nut. I actually found it with my hand before I saw it. There's a little room just down and left of the heater core where if you contort your body like a circus freak, you can see the bolt.
Step 5: There's a brace that kinda holds up the plenum, pretty much right beside Step 4, and it runs all the way to the front of the plenum (like the plenum is just sitting on this brace). It's connected to the frame on the center hump. Pull the carpet or flooring back, and you'll see the two bolts that connects it.
Step 6: On the driver side of the center hump, there's another brace, but the bolt is connected to the plenum. There's also a little small dowel that kinda lines the brace up on the plenum, so once you remove the bolt, be aware of that. I don't have a good pic of this, as I'd already put most of my dash back on, but here's the location.
This is my old evaporator core. Looks nice, huh.
I hope this helps some folks out. I've asked (and continue to ask) so many questions here, and you guys are awfully helpful. Just trying to do my part. Let me know if I've left something out.