In the FAQ, the lumbar repair covers more than necessary. I have repaired the broken screw assembly many times in my 4 Volvos (740 and 940). This is a stress fault in the design. The threaded portion pulls out of the nylon assembly due to stress from being tight(full support).
1. Remove the broken threaded piece of the assembly by turning the lumbar support knob on the seat counterclockwise.
2. Move seat full forward.
3. Put back as far forward as possible.
4. At the bottom of the seat back cover, there are 4 wire retainers. Clip them to release the top and bottom covers. Leave the wire in place.
5. Lift the seat back cover upward until you can feel the lumbar support assy.
6. Using a needle nose plier, you can pull the retaining pin in the assembly down, accessing from the back side of the square nylon piece in the assembly. You can also tap it out using a punch. I've found the pliers easier.
7. Once the pin is removed, the broken square piece is removeable.
8. Remove the nylon assy and pin from the new support bracket.
9. Insert the nylon screw assy into the mechanism
10. Insert the retaining pin and tap into place to hole the new nylon assy in place.
11. Test by reattaching the knob on the seat and adjust.
12. Pull the seat back cover back to its original position.
13. I use bent-nose small needle-nosed pliers to reach from the bottom seat cover, through the holes in the bottom and top covers. BE SURE TO GO AROUND THE RETAINING WIRES. THESE HOLD THE BACK AND CUSHION COVERS TOGETHER.
14. I use small wire ties, put the small end into the 90 degree pliers, then pull through the hole. Attach but don't tighten until all 4 have been put in.
15. Tighten the wire ties and move the seat to your normal driving position.

Additional. Now that you have the remains of a new assy and the broken nylon adjustment screw, you can repair the nylon end by drilling and putting a screw through to broken assy. I've had minimal luck with these holding however, but having a quick fix available is nice, and at no cost. The first time I did this, it took me about an hour and a half because I had to learn. I can do the entire job in about 15 minutes now. Very simple. If the nylon assembly was available separately, it would be ideal, but it's not.

Best regards,
Scott