seat heaters are cake. really not hard at all. ive not done them yet, but im going to in the next week or so and ive heard it won't be hard. it involves getting the covers off and putting in an aftermarket heater.
in regards to your backrest. take off that rear seat panel - easy - one screw and slide it down, you can roll up a small hand towel and put it in between the springs and the padding to give you better back support. made a difference for me and cost me... well.. a hand towel.
let us know what you do