Date registered: Dec 2005
Vehicle: W211 E320 Bluetec R129 SL500 - $1300 car ........ Ford F350 6l
Location: CA Bay Area
Mentioned: 9 Post(s)
Quoted: 1068 Post(s)
Your car is W123 body and you might get more precise answer in the model section.
From what I remember from the model -all plugs are connected by one power rail, so testing plugs require some disconnecting, however your problem seems to be power supply, meaning you need to seek 12v going into the plugs. That happens for few seconds after each ignition turn.