This is what I did, when I had the problem
My car was a 51 sports tourer.
Problem:- Niether the separate window or the main tailgate would open, but all other doors opened Ok (therefore fuse = Ok)
Step 1 - remove retaining screws (if you can call them that) for main interior trim on the tailgate
Step 2 - remove the 2 boot handles, be carefull, i used a flat blade screwdriver to help.
The liner will then drop out.
Step 3 - remove switches and check for any corrosion, clean as best you can. Spray with WD40 or similar
Step 4 - re-assemble and refit the switches putting silicone sealant around the interface between body and switch
Step 5 - Try the switches.
By the way, along the same section of the tailgate as the switches there are already 2 drain holes (rusty). Not sure if this is the Mod referred to.
I would also avoid using a pressure was around the switches as a precaution.
I did this 6 months ago and have not had a problem since.