Q: Where exactly is the location of the check/fill plug for the transmission fluid. on 1999 Audi A4 Quattro

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I know its under the car but I have failed to locate yet. thanks for any info you can provide.
(1) Answer
The drain/fill plug for most european vehicles is the same plug, and is located on the bottom of the transmission pan.
To change, remove the plug, remove the fill-tube from inside the hole, fluid will rush out, and then you can drop the pan to change the filter.
Once that is done, replace the filter, gasket for the trans oil pan, fill-tube, and drain plug/washer after pumping fluid into the drain plug hole with a specialty adapter and pump.
The vehicle must be run to around 35-45 degrees Celsius, and then the plug can be removed with the engine running to drain the excess fluid.
If you have never done this before, watch videos of the repair multiple times, and avoid the job if you are not confident.
If you need further assistance, let me know, otherwise, here is a list of certified shops in your area and repair your vehicle: