I finally figured it out.. Many thanx to those who contributed to this solution. I will outline what i found below, and hopefully help someone else fix this issue on their own. The dealer wanted $452 for a control switch and Tasca wanted almost $300 for it as well. I didn't even bother to ask for the seat motor price, and the representative simply said "remember this motor comes with the rail assembly, so it will be expensive". As if $450 for a switch isn't expensive already. Mind you, even though my Expi is a 2007, it only has 52k miles, and i just couldn't believe that a motor or a sensor could go bad so soon. Ultimately i found that it was a bad connection causing the problem. For more information, continue reading.
So this was my problem: 2007 Expedition driver seat slide would not move back and forth. All of the other functions worked fine (recline, lumbar, tilt, temperature and memory). The fist step i took was to check the fuses in the passenger side fuse box. Both fuses (#12 memory seat function & #2 power seat function, i believe) were intact. Then i proceeded to unbolt the seat and raise it just enough for me to gain access to the wiring. I didn't want to remove it, so i could check the connectivity with the harness connected. I also removed the seat control switch on the left side of the seat, but left it connected to the harness. I used a test light probe to check current coming into the seat control, thanx to the wiring harness you guys provided. I had current coming into the control, but then i checked the individual connectors while pressing the control buttons and the only one that wouldn't light up (show current) was the slide button. That means no current being sent to the motor. I decided to take apart the seat control switch to expose the PCB board. Pictures attached. I used a Multimeter to check for continuity between the points across the board and there was continuity across. However, some channels on the board overlapped other channels, I'm guessing to give the user the option of doing multiple functions at the same time (reclining and sliding for example). Not being able to determine if the switch was bad, I decided to move to the motor and check the operation. I pin-pointed the slide motor (there are 3 motors on this seat. slide, recline & tilt) and disconnected the plug to check for current. The wire color did change coming into this motor, as one member mentioned before. When i checked for current while pressing the slide button, none was shown, however, when i pressed the recline button, there was current going to the slide motor connector.
I went back to the seat control switch and looked for signs of rust, burned or broken connections, as mentioned in other threads and didn't find any. I did clean the connections just for peace of mind. I considered buying a switch, thinking it was inexpensive, and after speaking to the dealer rep, i nearly started crying, and decided to make sure and identify if the problem was the switch or the motor. I noticed that every time i pressed a button on the seat control switch, i would hear a "click" just before the movement, except for the slide function. This meant there were relays activating for all other functions, except for the slide function. So i started to look for relays in the seat. I found the "clicking" coming from a control module where all of the wires plug into, and then re-distributed to the motors & sensors. I pulled the module out of the pocket in which it sat and disconnected all of the plugs. I opened it up to expose the PCB board and saw nothing broken, burned or corroded. I saw 3 relays embedded into the PCB board, but they looked brand new. I decided to check the continuity on the circuit board with the Multimeter and when i connected the plugs back into the control module, i felt the seat move. I then pressed the slide switch and, you guessed it, it worked. I couldn't believe it. All of the plugs were connected when i first started to take everything apart, but i guess something caused the connection to fail. I'm not sure how this could have happened, being that this module is very tucked away into the seat and the plugs or wires do not become exposed to your feet because the seat cover does a great job of covering everything up. Though somehow, the connection was loose.
So to wrap it up, just imagine how many dealers or shops have been ripping customers off by replacing parts when a simple bad connection was the original culprit. I would like to mention that i am not an electrician, or a technician or even remotely capable of putting a circuit board together, but like i mentioned earlier, i was determined to fix the issue. So, if this ever happens again, i would recommend the following in the respective order, if one fails, move to the next step:
1. Check the fuses - 1 for memory seat function and 1 for power seat function.
2. Check all of the plugs under the seat and insure proper connection.
3. Check current with test light or multimeter coming into the seat control switch.
4. Check current leaving seat control switch while pressing the buttons.
5. Listen for relay "click" when button is pressed.
6. Locate desired motor and check the plug connector for current feeding into it.
Circuits are usually setup in the following order; Current from fuse to switch to control module to relay and finally to motor. Therefore the motor should be the last item to fail, unless there is an over load (track jammed, preventing movement & continuously pressing the button to force movement), then the process would be reversed and the motor will burn out first. Again, I'm not a technician or electrician, so perform all of this at your own risk. All I know is, this is what worked for me.
Again, thank you all for your help and contributions. I have attached a few pictures for illustration.
