This is weird,biff. I have same truck and same issues Lol. My antenna is also broken. The stealer, ahem dealer wants $87 for a replacement mast. Screw that. I did a quick search on fleabay. I found a bunch for @ $20.
As for the fender liner removal to get to the antenna unit, I removed mine last weekend to run some wires for my Hella 700ffs. It is not that bad. 3-4 screws at the fender edge and like 6 or so plastic screws on the inside. For you, it will be easier as you have removed the running boards. Those hide 1 last screw.
However, you don't have to remove the power antenna motor assy., to replace the mast. You just remove the lock nut at the fender, power on the radio which activated the motor assy and pull the antenna out. Simply feed in the new antenna and power off the radio and it will retract the antenna. Just re-install the lock nut at the fender and you done.
But, if you want to install a different/ fixed unit, you will have to remove the inner fender liner.
I will be replacing the power mast on mine as I want to be able to retract the antenna and not have it permanently extended
HTH:elkgrin: