Most of our users in those kinds of areas will keep the trees in a cold shed, or room etc... If the garage is not heated that would be great. Just watch out with the windows that it doesn't get too warm in the window. Solar effects and all that...
Ideally something in the 40-50 degree range is ideal for overwintering most junipers. They can take freezes though. But if it's going to freeze it's best to insulate your pot. I commonly do that by getting a strofoam crate, cutting a drainhole in the bottom, then putting mulch bark in it, and setting the tree and pot in the mulch, and then filling in above the soil line with mulch. It's fine to let foliage freeze... just keep it out of drying winds. It's the roots you have to protect most, along with the pot.
Dormancy is achieved on a tree when it gets in the 50's, so if you can keep the tree in temps at or below 50 degrees, it will get enough rest to be all ready to hit the growing reason at a run come spring.
Welcome to the family...
Kindest regards,
Victrinia