Where is the transmission drain plug on a 1993 Saab 900 Manual Transmission?

There isn't one. Saab deleted the transmission drain plug on the 900 in the early 1990s, claiming that the transmission oil was a "lifetime fill." If you want to change the oil, the old gearbox lube will need to be drawn out through the transmission dipstick tube with a siphon pump. Saab recommends 10W-30 engine oil as the medium of lubrication. Check your Owners Manual for other options. Note from a professional: It is not possible to properly drain all of the oil through the dipstick tube. It is preferable and recommended by SAAB to remove the stamped steel differential cover (8 bolts). With the car jacked up from the front, full draining is assured. The differential cover contains an oil "catcher" and filter which should be dismantled and cleaned--you will find bits of metal and junk here. Look for an excessive number of larger metal chips or "mud" which would indicate harsh use of the transmission and/or incipent pinion bearing failure. Be sure to use a new gasket when you put the cover back on. 10W-30 engine oil (petroleum oil, NOT SYNTHETIC OIL) is preferred.