just dont do like i did after i removed the second heater and found only one plug - instead of the 2 required. forgot about it... started bus, then wondered why there was water dripping out on the backside of the rear wheel arch.
for some reason it took quite a bit of water after i discovered the problem.
however, i did find out, that the radiator was only 1/3 empty, which tells me that the heater hose system being higher than the engine means that water is still in the engine block and that its water pump must be circulating water. thats my observation.