Does sound like you need to remove the object of his/her affection, which is the open garage, by shutting the door until he/she moves on.
Last year a young male robin decided that my work truck (which is parked outside in the open), would be a nice place to roost at night.
Every night he'd settle down on my drivers side mirror and poop up a storm, all down both sides of the mirror, then he'd step in his own mess, clean his feet off with his beak and rub his beak clean on my window.
I tried parking my truck in different places to confuse him. No dice, he'd find it and roost there.
He was very persistant and I had to deal with him for several months before he moved on. Probably after he found a mate.