The valve has power all the time and is grounded by the pushbutton controller when NO HEAT is desired.
The test is to verify the constant hot (fix if not) and verify the other side is grounded.
If you have these two conditions and you still have heat then you have either a bad coil in the valve or a stuck capsule. Check that the coil isn't open with an ohm meter and replace the capsule if it isn't. Its MB# is 000 835 06 44 and should be available from mercedesshop.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician