Right, it has to be installed in your "main home", not just any place you use as a residence, but the place you live most of the time and consider your main home.
And if this garage happens to be used for business only, you can't use this section 25C credit (but there might be an appropriate business credit, I have not looked that up). If the garage was used 50% for business and 50% as your home garage, you could apply 50% of the cost toward the credit.
There is no upper income phase-out for the credit. If your income is low enough–or you have other deductions and credits–so that you don't actually owe income tax, you won't get the benefit of the credit. It can reduce your tax, but can't give you a refund if you owe no tax to start.
