If the injectors are working, you should hear them "clicking" as the motor runs. If they "click" but no fuel is coming out, you have clogged injectors or a pressure delivery problem. With the engine not running but the ignition "on", you should hear a pair of injectors "click" 20 times as you move the throttle linkage from idle to full, rotate the engine to activate the next EFI switch point and pair of injectors. If the sliding blocks on the points are too worn, the points still may not work even after cleaning (might be able to bend it to get it working again.)

Is it missing at all load conditions or just at idle?

OE plug wire set for this MB is threaded post type, this is normal.

good luck,
Alan