I did mine, it's not a problem.
All you need are some needlenose pliers to remove the hog rings that hold the covers down. Then you pull the covers up enough to slip the heating pad inbetween the cover and the foam. I used some 3m adhesive spray to keep them in place. Then i bought hog rings and hog ring pliers to re-attach the covers. Note that you can only install pads on the flat parts, you'll see what i mean.

I used a kit from here: Seat Heater for Cars and Trucks from HeatedSeatKits.com

I went for the innovative heat "II" and if i had to do it again i would buy the "extreme" as it takes a bit of time for them "II"s to heat up. With a heavy winter coat on i don't feel it for 10-15 min. if im wearing a t-shirt i feel it in 3-5 minutes.

Make sure you get a kit that is wired in parallel like the HSK's i bought, as they will last a long time. Kits wired in series will die if there is one break in the element.