Ford's 4R70W automatic transmission was available as an option in the 1999 Mustang and the Mustang GT. If the transmission filler tube is damaged or if the tube pulls out of the transmission when you check the transmission fluid level, replace the filler tube to allow proper insertion of the dipstick -- both as a seal to prevent fluid loss and as a way to check the fluid level.
Park the Mustang on firm ground, put the transmission in "Park" and set the parking brake. Open the hood.
Find the transmission dipstick in the rear of the engine compartment on the passenger side. Grasp, twist and pull the dipstick from the filler tube.
Find the filler tube mount bolt at the top of the bell housing, where the bell housing meets the back of the engine. Of the two bolts at the top of the bell housing, the filler tube bolt is the one toward the passenger side. Remove the bolt with a socket and ratchet.
Chock the rear wheels. Position the floor jack under the front crossmember and lift the car. Support it on jack stands under the frame rails behind the front wheels.
From under the car, wipe the base of the dipstick tube to remove dirt and debris that might fall into the transmission once the tube is removed. If the tube is already pulled out of the transmission, cork the open hole with your finger while you wipe away the debris. Don't allow dirt to get into the transmission. Pull upward on the tube to remove it from the transmission. Inspect the bottom of the tube to make sure the O-ring seal came out with it. If the O-ring stayed in the transmission, fish it out with your finger or with a spring-hook tool.
Apply a light coating of transmission oil to the O-ring on the new tube. Position the new tube in place and insert the end of the tube into the transmission.
Lower the car to the ground. Install the filler tube mount bolt and tighten it to 30 to 40 foot-pounds with a socket and foot-pound torque wrench. Insert the transmission dipstick into the filler tube.
