For most vehicles that have a dedicated dimmer control wire, it's typically a light green or light blue wire on the vehicle's wiring harness. This wire is directly connected to the dimmer switch, which controls the illumination level of the instrument panel and other lighting elements in the vehicle.
However, it's important to refer to the specific wiring diagram or instructions for your radio to confirm the exact wire color and connection procedure. Miswiring can lead to damage or malfunctioning of the radio and other electrical components.