I fixed it! I am so happy, and I really appreciate all the help and encouragement I got from you guys! It was unlike anything I would have thought or expected.
As I said in my previous post, I got that green stuff out from the T in the black hose by the switches, but I still was not getting any vacuum on either end of the black hose. I then disconnected the harness (where all the lines - with the exception of white one - meet just above the gas pedal. I wanted to see if I am getting any suction out of the black vacuum line there, and I could feel it, it was pretty strong. At that point, I was 99% sure something is clogging up that black vacuum line on its way to the switches, as I could not see any leaks/cracks. So I stuck the wire inside from the switch side, and tried to push it as far as I could, though I could never do it all the way to the other end (roughly 2.5 feet). However, at one point, I fished out a bunch of that green glue substance as it got stuck to the wire. I connected all the wires back, started the engine, and everything - front and rear - was working properly!
I have no idea what that green stuff is. My guess is it originated at the front switch, first clogged up the T connection going to the rear switch, so that went out first (about 3 years ago). Over time, it clogged up this main vacuum line, so the front went out (2 weeks ago) because there was no suction left. Like I said, I would have never guessed anything like that in my life. It had to come from the front switch, since there was that substance right on the switch connector to the black line. It could not have come from the other side (harness). Anyway, it looks like I did not have to buy neither rear nor front switch (anybody interested, I will sell it to you for much less than I paid Ford couple days ago), but I am sure happy to be able to fix this long term, annoying problem. I just wonder if I should replace that piece of black line, or hope that I got all the gunk out already.
Thanks again for your help, I would not have done it without this forum. BTW, I also fixed another annoying problem - internal lights on all the time - by spraying WD-40 into all the door latches, as recommended on this forum. | 
