at 11:28:18 AM
Your concern is definitely justified! There should be no penetrations from the garage to the living space of your house - it is a huge safety issue not to mention the fact that you are trying to heat the world!
The fix is to open up what you have to from the garage side and remove the duct back as far as you can - all the way to the main duct if possible, and install a proper cap or cover over the hole. Any box store will carry round caps in all sizes or sheet metal pieces to cover the main duct. This should be screwed and properly sealed (foil tape). You would then check and repair the vapour barrier (should be on the house side of the wall) install new insulation, and cover with at least 1 and maybe 2 layers of 5/8 fire code drywall - duplicate what is there. This would need to be properly sealed with tape and at least 2 coats of compound.
If you are not comfortable taking this on, you could put your requirements in the "post your projects" section and get some local contractors to contact you. You should deal with this as quickly as possible.
Good Luck with it!
Kettleby Handyman Services