Geez, a simple question. You'd think someone would know.
Well, I do, kinda.
The diagram that you're seeing is for a pre-87 Stang. I should know, I have an '86.
For 87-93, the parking brake handle has a spring that
should keep tension on the brake cable. Also, as people have said, the star wheel adjuster in the drums must be working and have the correct adjustment.
With the above said, there's an equalizer right below the parking brake handle that transfers the force from the single cable from the handle to the two cables that go to the back wheels. There is often the ability to have an adjustment there. I do
not know how that equalizer bracket and cable changed over the 87-93 years. However, I do know that a many years ago, a Fox Stang that I found in the junk yard had an "adjustment nut" on the cable that went from the brake handle to the equalizer. If it was really an adjustment nut, or if there was juts a long section of threaded rod at the end of the single cable, I can't say for sure. What I know is that the Fox that I took the equalizer bracket from didn't have the equalizer bracket all the way back on the threaded rod section.
Also, for a properly adjusted and working parking brake system, with
stock size and type tires (width and not race tires), basically any car should be able to lock up the rear tires with the parking brake at speeds under ~5mph. Most will do it up to 10mph.
So, you have a number of things to check, and that could be wrong.
Good Luck!