How do you repair the radio in a 1985 Cadillac DeVille if the seek and scan are not working?

Take it to a radio shop.

Just replace it with aftermarket. Wiring

1985 Cadillac Seville Car Audio Wiring Diagram

Car Radio Battery Constant 12v+ Wire: Orange

Car Radio Accessory Switched 12v+ Wire: Yellow

Car Radio Ground Wire: Black

Car Radio Illumination Wire: Gray

Car Stereo Dimmer Wire: Brown

Car Stereo Antenna Trigger Wire: Pink

Car Stereo Amp Trigger Wire: N/A

Car Stereo Amplifier Location: N/A

Car Audio Front Speakers Size: N/A

Car Audio Front Speakers Location: N/A

Left Front Speaker Positive Wire (+): Tan

Left Front Speaker Negative Wire (-): Gray

Right Front Speaker Positive Wire (+): Light Green

Right Front Speaker Negative Wire (-): Dark Green

Car Audio Rear Speakers Size: N/A

Car Audio Rear Speakers Location: N/A

Left Rear Speaker Positive Wire (+): Brown

Left Rear Speaker Negative Wire (-): Yellow

Right Rear Speaker Positive Wire (+): Dark Blue

Right Rear Speaker Negative Wire (-): Light Blue