Where do you add transmission fluid to a 1999 Ford Mustang with manual transmission?

A 1999 Ford Mustang with a manual transmission doesn't have a dipstick to check or add transmission fluid. The transmission is generally considered to be a sealed unit requiring no routine fluid changes or top-offs unless there's a leak or damage. If you believe there's a problem with the transmission (e.g., difficulty shifting, noises), you should consult a mechanic. Trying to add fluid where there isn't a readily accessible filler port could cause damage.