I'm assuming you're talking about the metal trim in front of the top of the rear glass?

Yes, there are brackets up under the headliner.  The trim pieces go much tighter up there than you'd ever think, and finding the holes is a royal PITA.  My method has been to locate them without the trim pieces in place using a pick, etc. then cut a small circle pout of the headliner material so they are easier to find.  You need to push the headliner up tight to them when you do this.  Then install the trim pieces using a pick, etc. to locate the holes.  The screws for these panels are also longer than you'd think.  They aren't the short screws used for all the other trim, they're more like 1.25" long. 

All the rear trim in a fastback is a pain.  Take your time and get it as right as you can, because you don't want to go back and do it twice.  But, you also have to understand that fit and finish in these cars isn't what it is on today's cars.  Your best is probably better than the assembly line's efforts because you have time on your side.  You may need to use a shim or two, or a whole stack of them in some spots to get stuff the line up right.  They were there from the factory, but usually have been lost over the last 50 years.

They are being reproduced, but I don't know if you can get them piecemeal, or if you have to buy the whole kit.  Here's a link to one site that carries them, I would assume most of the bigger parts houses also have them, as they are built by Dynacorn and most places carry their parts:

https://www.stang-aholics.com/i-24185438-67-68-mustang-fastback-interior-body-bracket-kit-42-pieces.html

If they are not available separately as reproductions you may want to try Tony Branda.  They have a lot of used parts as well as new.  Really good people to work with.  I actually bought all of my reproduction fastback trim panels from them, which are Dynacorn repros.  They are plastic, not fiberglass, and not 100% show perfect, but high quality and fit well.  Here's a link to Branda's site:

http://www.cobranda.com/