"Under 32 degrees liquids freeze. Diet soda is very similar to water so will freeze near the same temp (30-32 degrees).. regular soda can stand temps a bit chillier but at 16 degrees will probably freeze too."
There are two things at work here that can change the situation. Firstly, an enclosed garage that has contact with a house will generally not drop to the ambient outside temperature unless the door is open, so in 16 degree weather the interior of the garage is not likely to be nearly that cold. Also, if you'll pardon the pun there are degrees of freezing to consider. Soda in a can has to freeze hard, crystallizing all the way through to expend enough to deform the can or damage it. Very near its freezing temperature, soda (even diet soda) tends to be slushy because it doesn't freeze uniformly until it's well below its base freezing temperature. So given these two things, I wouldn't be concerned that soda in the car would freeze hard enough to burst.
Betelnut, the ultimate easy test for you is to go into your garage sometime when the weather is well below freezing. If you can't see your breath, or if you don't have to scrape your windows when your car is in the garage, it's not likely that the inside temperature is below freezing.