Given that I'm in the mountain time zone I didn't have a chance to respond on our company forum before you posted to vb.org so I'm replying both locations.
To answer your question there is some code to do this. You just need to edit the dto_garage.php and look for the last use of require_once near the top of the file. Immediately after that paste in the following:
PHP Code:
if(!$vbulletin->userinfo['userid'])
print_no_permission();
save the file. Now if anyone that isn't logged in tries to access any of the functions supported in this file you will get a message and login prompt.
In our Garage Plus product we offer the option for the site member to determine if each of his garages is private (meaning no one can view it), public (everyone including visitors), or just friends in his buddy list can view his garage. Based on the setting it will determine whether or not its displayed in the index of garages, garage drop downs displayed in posts, etc.