Here is the link to the post at gamefaqs which explains everything...
http://boards.gamefaqs.com/gfaqs/gen...topic=18103628
Here is the map to the hidden interior universe...
http://gtadomain.gtagaming.com/hiddeninteriors.html
And finally here is the video on how to get to the hidden tattoo shop...
http://www.edgefiles.com/files/17888.html
All these links are on that first original post at gamefaqs which explains EVERYTHING you need to know, but I highly suggest you watch the video because getting to the hidden tattoo shop is VERY VERY difficult....