On my 89 ford f350, I am in the process of installing a stock oem radio in it. however, I am not getting constant 12v from the light green/yellow wire like it should according to the wiring diagram. It gets 12v on the acc. wire when the key is turned and the ground wire is connected, and the radio definitely works. How does this wire come off the battery?

 

If it's getting VDC only when in accy, I would expect Ford changed the wiring for you. Have you checked the others for constant?

It will be fused, but I don't know what it would be fused as. So battery to fuse block to radio would be my assumption.

 

I actually worded that worng. the light green/ yellow wire that is supposed to get 12v all the time never gets power. The yellow/black wire, on the other hand, gets power when the key is turned like its supposed to. If I went the route of running a wire from the battery for it, what amp fuse should I use?

 

A 10 or 20A fuse should be plenty fine. This fuse is there to protect the wire not the components, so you need enough fuse to power the radio, and blow if the wire is shorted to ground.

Because it never has power, I'd suspect you have a blown fuse. Don't trust the eyeball method, fuses can blow but look okay. Break out the DMM and check them all.