OK the real story here is who is forcing you to keep this in the garage? Eh? Eh??

I live in Cambridge, ON (45 min west of Toronto). A couple years ago I was living in a townhouse and I kept my Galaga machine in the garage. It survived, but I will never do that to a machine I care about ever again. The moisture rusted some of the grills on the back door, and also rusted a bit of the transformer and the clips on the fuse block.
Game still works, but every time I see that rust I just wanna kick myself. (And keep in mind, this was in a garage used as storage. It's not like we kept opening the door all the time and parking a wet car in there!)
So, as was already said above... it will be fine, provided you let it warm up and dry out thoroughly before you turn it on again. But if you want to avoid rusting, you might as well wrap the whole thing up tightly in plastic as a precaution.
~Ray B.