The SOHC M103 engine block coolant drain plug is back near the #6 exh. and uses a 19mm socket to loosen. 6 point socket is better as it get a better grip.
Don't know about the '98 mentioned above, but on many radiators fitted to 103 motors, the radiator drain plug is fitted on the front of the radiator - passenger side and can be a real bear to get to. On the 103 motor, it's easiest to drain the radiator by simply pulling the hose off the lower inside of the radiator - passenger side. This drains the rdaiator and the expansion tank in one shot.
Some will argue that removal of the radiator drain plug is better as it's lower than the hose I just mentioned and I would not argue with that, but again, getting to it can be very interesting in some cases.