Does the car run? Ground the interlock inhibit wire to battery negative. If light stays on when the circuit is grounded then your instrument cluster is bad. If it goes out check the circuit between cluster and charger. If circuit is good then your charger is the problem. Charger should send a ground signal when the extension cord is not plugged in. The inhibit wire is pin 13 GN/BK wire on the 20 pin white instrument cluster connector. If the cluster is bad make sure voltage is not being sent on the charge inhibit circuit as that will fry the cluster. I have the shop manual which would help if you don’t have it. Send your email if you need it.