I began to draught a reply here based upon a bad earth scenario (which altered de-dounce timing may fix but still not completely disprove IMO) and in the end gave up (too many potential causes). Personally I'm with the higher than 0v earth potential at the stop light (bad earth) explanation.

The reason I gave up was I because I needed to make too many assumptions about the empeg's sensing and electrical characteristics such as:
1. Does the empeg utilise a potential divider to sense
2. Is that smoothed by a capacitor
3. What is the input impedance
4. What is the sense threshold voltage

It goes without saying that understanding the electrical theory on gounding problems is beneficial but in itself is not a cure practically speaking. Personally and admittedly OT, I'd benefit from an answer to q.4 (sense voltage threshold) to help cure a sense 'problem' I have with partially powered headlights in sidelight+engine running 'mode'.