Your dad will hit the ceiling if you leave the door open to the garage. The utility bill would be ridiculous.

I don't think you should attempt this until you could track the temps, both highs and lows, and see what they are. I don't know that fans blowing the hot air from outside into the already hot garage is going to do much for you. I think in the summer you are going to heat dangerous numbers and though yall don't have much of a winter I think you would also have a heck of time keeping the garage warm enough on cold nights. All of this will be even harder if yall actually park cars in the garage and the door opens several times a day.