2000 bonneville instrument guages go nuts or may work properly, a/c may come on or maybe not (fan always works, though sometimes the passenger a/c blows cold and the drivers won't), and courtesey lights may or may not come on. Replaced ECM with no luck. Thinking BCM? But there are more than one? My daughters car, in Florida, with summer and luv bugs, this is a must fix for the a/c.
Suspect IPM located under left side dash. First check IGN-# relay in rear panel under left rear seat see diagram. When AC not operating check for voltage across HVAC fuse if none tap on relay if AC comes on replace Ign-3 relay