If you don't want to mess up in the engine bay you can drain out some coolant from the radiator. The draining plug is located in one of the lower corners on the radiator. You can fit a hose on the plug and collect all coolant. Drain about 0,5 litres and there won't be any coolant left in the termostat housing.
If the coolant is clean you can reuse it when done.
'87 MBenz 190E 2,3 8vlv