I have retrofitted many things in my car that it never came with http://homepage.mac.com/drsunilpatel/PhotoAlbum14.html

If a few pennies saves the company money then I am sure the company will leave prewiring out. Multiply that times the number of cars they sell. I installed vented/heated and orthopedic seats in my car. You can definitely do it though. It's not a very hard retrofit.