I don't have the electrical answer, but there are three or more rubber grommets through the firewall into the interior with unused holes that you just cut off the tip to use.

"What's the point?" More feedback to save gas and reduce his carbon footprint.

"I got a ScanGaugeII and I am very happy. My coworker bought one also and he likes it very much. It plugs in super easy, into the OBDII connector." A 95 E320 does not have OBDII??