6-way power drivers seat backrest broken

Likely depends on the seat. But basically, you should be able to turn the upholstery away enough (again, HOW depends on the seat) to see what's wrong. Years ago we had a couple 1980's TBirds, and the backrests were pretty flimsy. Very thin sheet steel that got its strength from its shape. A couple of them cracked and got wobbly. I was able to get the covers and padding off, and have the metal "repaired." I forget if it got welded, brazed, or if adding little repair patches was involved.

If you're talking about the "adjustable lumbar support" as used by Ford, where you turn a knob and the shape of the backrest changes, then YES, only the part can be replaced, around $100, or you can do what I did in my daughter's Explorer and tie it back together with a few ZIP Ties. For part of it, I got the ZIP ties with screw holes to put them where I needed by drilling the frame and driving self-tapping screws.