Maybe I shouldn't reply to this, since I'm in z8 and have not gardened in your climate. But I would dig a hole in the garden, stick in the pots, backfill and cover them with a deep mulch (straw, hay, etc) to act as insulation. An unheated garage would be very cold, even with some insulation around the pots.

Oh, I can definitly tell you not even close to my climate LOL, you see, Thats a great idea, but, the ground is frozen and even covered with a half inch of snow, Were having highs in the mid 20s and lows in the teens. It is after all December in Wisconsin.

Upper Midwest Rhododendron Chapter:
"The potted plants can be placed in a garage, potting shed, or basement provided there is sufficient light to keep them alive. They cannot survive in the dark. Artificial lighting can also be successful, much the same as germinating seeds. In all cases, water as necessary, not excessively.
Generally, storing the plant in a potting shed with sufficient light, unheated, will gain 15-20 degrees, as compared to the outside temperature."