You know what im not sure about a drain plug, i had the drive shafts out when i removed the box in my ST170 and the fluid drained out this way. A HUGE MASSIVE TIP though is put you gearbox fluid in a bottle with a squeezy top, you have to fill the box through a hole that is horizontal so the best way is to do it via this method. Ford gearbox fluid only though, dont let people fool you with royal purple fluid or anthing like that. Ford stuff is the best.