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Along with the accelerator pedal plug, two additional wires need to be integrated under the dash. The red ignition switch wire should be tapped into the ignition harness for keyed 12 volts. On older GMs this will be a thick orange wire. The gray MIL wire is the trigger for the check engine light. It supplies the ground for the circuit when triggered. We’ll probably tie this into a stock idiot light, though we’re tempted to put in an actual check engine indicator. The OB2 port just needs to be accessible, so anywhere under the dash is fine. We’ll likely mount it on the backside of the radio.