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Another thought, still a guess, is that corrosion has broken some circuit (opened by corrosion) and then the devices are feeding back where they come. This thought comes from the age of the vehicle (same age as my Volvo sedan).
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Thanks and good luck with your vehicle
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The turn signals work when the bright lights are on and pull back on the switch. On one occasion the brights were turned on and the radio went off.
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The turn signals work when the bright lights are on and pull back on the switch. On one occasion the brights were turned on and the radio went off.

2004 Chevy Colorado wiring

In addition to ignition, power and monitoring gauges and lights, today's motor vehicles feature computerized engines, transmissions and braking systems, electronic mapping systems, Bluetooth and Internet connectivity, sensors that detect other vehicles, motorized seats, and digital audio and video systems; several manufacturers are producing electric and hybrid automobiles and self-driving cars.