I work out of a heated shop but i pull the car out to dry. I place two small fan heaters (not expensive ) in the rear seat on a small towel. I then slightly crack the windows, start the car and crank the heat on high. Even with these small, cheap heaters the heat will really crank up inside, about as high as im comfortable with. So, big expensive heaters or blowers arw absolutely not neccessary and could actually be harmful. You can tell your customers that you will be running their cars for around 20 minutes. Ive never had anybody oppossed to this and it works extremely well.