The power should be coming from EHC like you said. (Runs through EHC, not controlled by it) Remember, I can't see it. The red wire should have 12V. So test it with fluke. Let us know you are getting 12 v.
The ground wire should have continuity with negative post of the battery. Very low resistance. Check it, unhook positive first, then check with multimeter.
Then perform test on speed sensor, post all those readings. Test as close to the speedometer or sensor are you can.
Make sure battery is unhooked when testing resistance, can fry your meter.