1. Turn off the car and make sure that the keys are out of the ignition to ensure that the car will not be turned on while you are working on it.
2. Put the hand brake on the car and then place a large log or brick behind one of the back tires to avoid the car sliding back while you are working on it.
3. Put a jack behind one of the front tires of the Audi A4 and jack up the car about 3 feet from the ground and then slip a jack stand to hold that side of the car up. Repeat this step on the other side of the vehicle.
4. Slide in under the car and look up into the hood area of the car until you find the A4's gear box. The gear box will look like a round, metallic cylinder.
5. Slip a 17mm Allen wrench behind the gear box and unscrew the fill bolt. Slide the fill bolt out completely once it's unscrewed. If any transmission fluid drips out of the hole the fill bolt was in, then the transmission is full and doesn't need any more fluid added to it.
6. Fill up a fill bottle with transmission fluid for your 1999 Audi A4 and then inject the transmission fluid in through the hole where the fill bolt used to be until some of the transmission fluid seeps out.
7. Replace the fill bolt and tighten it with the same Allen wrench.
8. Slide out from underneath your A4 and carefully lower your car back to the ground.