Can't get fault codes on 87 420SEL

Help! I've read the artical on this site pertaining to checking the fault codes with an oscilloscope. I have the MB service manual and it eludes to an "X92" connector near the battery on the passanger side of by the firewall. I cannot find any such connector. The only connector I have is a round connector on the drivers side fender. I've connected the scope to pin 3 and ground clip to positive on battery and the ignition turned to on (just before cranking) All I get is a constant square wave 12V. Even after grounding pin 3 for 2-4 seconds. Am I using the wrong connector? Have I got it connected incorrectly? I'm trying to use the scope so as not to spend the $350 for the code reader.

The only time this signal changes is when I start the engine, then the square wave slowly becomes a flat line as the engine warms. I think I'm see the on/off ratio readout of the lamda system. Any help would be greatly appreciated.