Problem: right now the seat is as far back as possible! I can feel around and I think I know what you're talking about, but I doubt I'll be able to do anything with it.

Actually this will only make the access to the area behind the seat more difficult, you should still be able to raise the rear of the seat and lower the seat front to get at the underneath of things.

If the electric motor has failed you’ve got a hard road because the rear two bolts will be hard to get at even with the seat rear elevated. The outside bolt can sometimes be accessed from the side using a swivel socket. The inside bolt is harder, you can sometimes get at it with a ¼” drive 10 mm socket on a T handle with it slid all the way to one side. The important thing is to just get the bolt to break free without stripping it then you can take and hour to move it 15 degrees at a time! A Gear Wrench is great if you can get it on the bolt head. Worse comes to worse you can grind the bolt head off using a die grinder and a good carbide burr.

Once you get the seat out you’ll need to get the seat base off the seat bottom which will require you in this case to figure out which electric motor drive the forward/back mechanism. Then you need to disconnect the drive cable at either the motor end or the mechanism end by removing the spring clip that secures the cable sheath. Once you get the cable disconnected you will need to take the actual drive cable and insert one end into the square cable receptacle at the mechanism and using something like a pair of small vice grips to grab the other end of the drive cable, rotate the cable and operate the mechanism to move the seat base forward or backwards to line up the access hole in the seat base to the large Philips head screw in the center of the seat bottom. The large Philips head screw is secured with blue loctite and usually requires some force to break free. I use a 3/8” ratchet with a 5/16” hex bit adapter and a large 5/16” Philips bit from a manual impact driver set you can get at AutoZone for $10. Of course you can also use any large regular Philips screw driver, you just want to avoid stripping the screw head down in that hole! With the center screw removed on both sides, work the mechanism forwards and then backwards to access and remove the other large Philips head screws at both ends of both sides. These six screws attach the motorized seat base to the seat bottom, unplug the wiring that goes to the two electric motors in the seat back, the recline function and the headrest function.

Now you can access the electric motors of the seat frame if you’re going to swap out a motor. Or it is probably easier to just swap out the entire seat base if you have one available.

Then place the seat base in position in the car, attach the four connections at the front and verify that the seat base operations all function properly. If everything works then you can attach the seat base to the seat bottom, remembering the seat back electrical connections and reinstall your tested working seat!

“The passenger headrest may have a relay issue-- I can hear the motor enguaging for "down" but no response for "up", on both the old and new seat adjust switches.”

With regard to the headrest function, if you can hear the electric motor operating when operating the switch the problem is most likely the plastic receptacle “key” part of the operator track which has failed. This issue will be most easily addressed with the seat out of the car, remove the seat back’s two small Philips head screws at its bottom and slide the seat back first down and then out. You can then easily see the headrest operator and its drive motor. This plastic part breaks and then functionality is lost, there are some quick fixes and replacement of the small plastic part will fix the problem permanently. If for some reason it’s not this usual problem it could be the drive cable from the electric operator to the mechanism, in the seat back all these parts are open to see and a problem will be apparent when you look at it.

Are the seat/track/motor base units different for the driver/passenger side? Some of the parts I'm looking at list driver/passenger, others do not.

The seat bases are basically the same, the electric motors are the same, the controls are not the same. The driver’s seat has the “master memory control unit” and the passenger side does not. You can use either seat base but would need to swap the controller onto any seat base that will be used for the driver’s side application. The one thing that I don’t know for certain is if the outside of the passenger seat base and the outside of the driver’s seat base are exactly the same, and I mean that only in respect to the openings in the seat base outsides that are there to accept the plastic prongs which attach the plastic trim to the seat base. I would be very surprised if they are not the same but I would check and get the plastic in place before I knew for certain. Good Luck!