Through the check/fill plug in the center of drain plug
from the transmission dipstick use some adaptor so you can drop your oild inside.
In the dip stick tube on the left side of the engine toward the back. It should be marked TRANSMISSION.
You can add transmission fluid to your 1969 Mustang transmission through the dipstick holder. Using a funnel will make the job easier.
In the fluid chamber
Into the transmission fluid reservior
You add fluid the same place you check the fluid level.
The dipstick cover is labeled Transmission Fluid. Transmission fluid on a 2000 Ford Mustang is added in the neck of the dipstick tube after the dipstick has been removed.
Unless you have a LEAK you shouldn't have to add automatic transmission fluid on a regular basis
It usually goes in where the dipstick for the transmission is.
Check the automatic transmission fluid level. If it is low add enough to reach the full mark. If it is not low seek professional help. If you have to add fluid use only Genuine Honda automatic transmission fluid. Use no substitute.
Get a lube shop to do it.
4 quarts