No, you should not install it in an unheated garage. The system must be installed indoors, in a location sheltered from environmental damage.
Fix: Find an indoor location. Under a kitchen sink is standard. The feed water temperature must stay between 40°F and 100°F (4-37°C). Water freezing in the lines or housings in an unheated garage will crack and destroy the system.
If that didn't work: If you have no indoor space, the only alternative is a conditioned, insulated space like a heated basement utility room that never drops below 40°F.
Heads up: Also avoid installing it under direct sunlight or near any heat source. Temperature extremes are the enemy.