You'll need the lordosis bag, the switch, and the wiring harness.
You can grab a junkyard seat for these parts.
Remove seat and the back cover. Install lordosis pad behind the spring mesh assembly. Run wiring harness down and tap it into the 12V supply (Big thick brown is negative and big thick Red is positive) of the mating harness. Use a dremel to punch out the cutout for the switch. Reinstall and you're done.
The hardest part of the whole thing is removing the plastic piece where the switch goes. Be patient and you'll be able to find all the correct releasing tabs.