If you don't like my response, oh well... you asked for advice/opinions so you're going to get them and it really bothers me how you resort to sticking him in the garage because he's doing what he KNOWS to do.
You make it sound like he doesn't get hardly any attention at all. I really can't understand or find the reason why you even have him. You've changed it so many times he probably doesn't know where the heck he's supposed to do his business, he's completely confused.
I think you really need to pay attention to the way you're doing with him, if you had a child (which is how I consider my girl) would you put him/her in the garage or a separate room if they had accidents? He's only going by what he's been taught/trained to do so I really think the one regressing is you.
Don't know if this is too blunt, hopefully not
.