cruservic,
buy the extensions as well if you want the buttons mounted on the dash. Then review the install locations for the buttons to determine your preference and go from there. I did mine recently and my suggestion is to take your time, not need the car the next day just in case. Lay everything out ahead of time, lay it out again before you cut any wires (if needed) and before you put anything back in to place, test the heaters. Like they always say, measure twice, cut once.
I didn't used to have the patience for this kind of method, but I have found ways to keep calm during difficult projects. I suggest anyone attempting any larger projects find a happy place to go during the work...

mmmm animal crackers.....
GJ