Can I put an HVAC vent in my garage?

My attic extends out over my attached garage. My air handler unit is in the attic along with all of my other ducts. I have an open spot on my air handler where I could add a duct. The garage portion of the attic is not currently insulated, but I plan to rectify that. Basically I have heard that its not a good Idea, and may be against code, due to automotive fumes getting into the HVAC system. Anyone know the truth? 10 points to the person with an intelligent answer.