Read the circuit before touching it
Identify power source, fuse, switch or relay, consumer and ground on the correct model-year diagram. Then confirm where voltage is present under load. Continuity on an unloaded wire can look perfect even when corrosion prevents useful current.
Grounds are measured, not assumed
A connection can look clean and still drop voltage. Measure between the consumer ground and battery negative while the circuit operates. Clean the correct point, protect it and retest. Adding random extra earth wires can mask the original fault and create future confusion.
SI board and ageing electronics
Failed rechargeable batteries on the service-indicator board can produce erratic instruments and board damage. Inspect rather than repeatedly resetting the display. Check Control faults also require circuit testing: the warning panel reports what it sees, but the cause may be a sensor, connector, lamp specification or supply.