I have a Proheat XL900 that I am going to install. I don't really have space for it in the normal A/C bay, so I am planning on putting it in one of my luggage bays (where the water supply is so that it can also help keep the water from freezing).
But I see that the Proheat needs, of course, fresh air intake. Is there enough leakage into the luggage bays to get fresh air for combustion, or do I need to vent it?
You definitely need to supply a source of fresh air. If you have a couple square inches of holes throughout the bay then you should be good. If not, add a fresh air intake.