...as in, where do I stick the electrodes of my voltmeter? There doesn't seem to be any place to stick them at the plug. Can I get a contact at the ecm?

 

Sometimes I use needles to backprobe the plug and alligator clips on the voltmeter. Be sure to unclip the weather protector.

 

Yeah, you'll need to rig some patch-cables so that it's still connected across all 3 points (5V+, ground, and reference signal back to the ECM) but you can still probe into it. I did this years ago on earyly TPI motors before scan tools were widely available (and nobody had even heard of a laptop computer!) You want to measure voltage between the reference signal line and the ground.

Nowadays I use a scan tool for stuff like this- like to see what the ECM is seeing, not a voltmeter gague reading that could be slightly off.