Try replace at heater valve and the heater hoses first since they are cheap! Sometimes, heater core leaks, which is not leak inside the cabin. If replaced heater valve and hoses, still see water leak out on the back of the engine area.... I will say heater core leaking.

Replace heater core on 190E is easy... about 6 to 7 hrs job. Just need to take out instrutment cluster, steering wheel, whole center console, dash. Then, only 4 nuts to hold up the heater box. Unclip the heater box and you will see the heater core.