Well, I'm not sure. I agree with you that the garage probably would not be a great place for them. But that breed doesn't do great outside either.My dad would let them in the garage, but it feels like it would be so depressing for them in there. There’s no natural lighting and it’s cold in there. What do you think? Better than the muddy outside?
Have you watched Lennon the Bunny? It's a really great youtube channel, and it focuses a lot on keeping your rabbits indoors. Maybe you could use that to get them inside?
If you absolutely can't convince your dad to bring them indoors, then maybe if you post some pictures of your current set up, we could come up with some ideas to make it a little better for them.