Just thinking out loud. Saw elsewhere on the internet that the exhaust of an HRV should not be directed to a garage.
I'm in the midst of designing a system for a new residence in Maine and think the addition of some heat by exhausting to a attached garage (that has full eave vents) is NOT a bad idea...the inlet would be seperate and well away from the garage.
Ducting an HRV exhaust anywhere BUT outside is a bad idea... you go right ahead if you don't mind exhausting moist, stale, possibly smelly air (if you're venting from the bathrooms) into your garage. Expect mold problems and a rusty car if you do this. Just my 2 bits