* Automatic Transmission: The transmission fluid is added through a dipstick tube located on top of the transmission. This tube usually has a dipstick with markings to indicate the proper fluid level. You'll likely find it near the back of the engine compartment, somewhere around the transmission itself.
* Manual Transmission: Manual transmissions on 1965 Mustangs generally don't have a dipstick. They're typically filled through a separate fill plug located on the side of the transmission case. You'll need a manual for your specific transmission to identify the precise location of this fill plug. Overfilling a manual transmission can be very damaging, so it's crucial to consult a repair manual.
In short: You need to know what type of transmission you have (automatic or manual) to know where to add fluid. A shop manual specific to your 1965 Mustang will be the most helpful resource to pinpoint the exact location. Looking at diagrams in the manual will greatly assist.
