I think you are forgetting they are in an insulated box inside of a cardboard box (double insulation) with heat packs....and the volume of water is significantly smaller this easier to heat/keep warm. The boxes are also stored at ~70 degrees when in a warehouse or plane so not always in the elements of the truck. The corals also only go through the fluctuation for up to 40 hours or so...not for 6 months.
A glass or acrylic tank is not a good insulator and with all the open water surfaces the heat escapes quickly. Opening the garage door in the winter basically exchanges all the air in the garage in 2-3 minutes with cold air since warm air can rise higher outside. Remember that temp changes (and acidification of course) are causing wild coral reefs to decline and stability is key...I wouldn't chance it IMO.
Also, I doubt carbon monoxide, etc. from the car would be good for the tank either, again, open surfaces allow a great area for a quick diffusion point of impurities into the tank. Think about how many tank crashes happen due to household cleaners being used too close to tanks, again, not worth the risk IMO.
I also doubt that multiple heaters would actually keep the tank warm...if the garage is like mine, which is insulated with a really good insulated door brick exterior and attached to the house, pop and water sometimes freezes in the winter....tanks are usually in a 65-75 degree house, so the heaters are meant to keep temps on a tank inside (within 10 degrees or so) not outside temps.
But ultimately it is your decision of course...just my 2 cents from my experiences and others I know....I am not an expert on this at all. Most people that do this (that I know) have their entire garage dedicated to their tank(s), not a portion.
