I picked one up last fall. Went to a salvage yard, that I have visited for about 23 years. Told the owner what I was looking for, and he said to go look in a bunch of the old delivery trucks, that had come from a local potato chip company. The trucks look like UPS trucks.
They had aftermarket, 2 speed heaters added, right next to the drivers seat. The added switch was on the dash. 4 screws, 2 hose clamps, and the unit was out of the truck. I think other companies, useing the same type of truck, may well have added these heaters also. --- John