Need help identifying radio wires

[67GMCcampercruiser]

The radio doesn't work in my 1975 Jimmy. I tested the wires from the harness and there is no power. There are 3 wires black yellow and grey. I assume black is the ground, the other 2 are power and power for the light. Is that correct? Planning on wiring directly to the fuse box and ground to the firewall.

[ray_mcavoy]

Yes, the black wire is ground, and yellow is switched power (should be live whenever the key is in the on/run or accessory positions ... if not, check the fuse). The grey wire is for the dial light and is connected to the dash light circuit (voltage on this wire will vary depending on where the headlight switch knob is rotated to brighten/dim the dash lights).

[67GMCcampercruiser]

Thank you.