You have it right. Use a 12 volt test light to see if there is power between the gray and brown wires when the key is on. If there is power there, you may have got hold of a bad radio.
1990 Toyota Celica Car Stereo Radio Wiring Diagram
Radio Constant 12V+ Wire: Blue/Yellow, connects to yellow
Radio Switched 12V+ Wire: Gray, connects to red
Radio Ground Wire: Brown, connects to black
Radio Illumination Wire: Green, do not use
Radio Dimmer Wire: N/A
Radio Power Antenna Trigger Wire: Black/Red
Radio Amplifier Turn On Trigger Wire: N/A
Front Speakers Size:
4? Speakers
Front Speakers Location: Dash
Left Front Speaker Wire (+): Pink
Left Front Speaker Wire (-): Purple
Right Front Speaker Wire (+): Light Green
Right Front Speaker Wire (-): Blue
Rear Speakers Size:
5 1/4? Speakers
Rear Speakers Location: Side Panels
Left Rear Speaker Wire (+): Black
Left Rear Speaker Wire (-): Yellow
Right Rear Speaker Wire (+): Red
Right Rear Speaker Wire (-): White
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