1989 ford wiring problem?

Hope someone can help. 1989 Ford E150 5.0 no A/C, aftermarket Sony CDX-GT240, no wiring modification, just used the factory plug connected to the radio's adapter plug. This radio has worked fine for about 4 years but, while driving the other day, the radio stopped working as if all power was lost. All fuses are fine. Ford's wiring is yellow w/blk stripe 12+ switched, green w/yel stripe 12+ constant. Checked for voltage at factory plug and found constant 12+ at green w/yel stripe, but no voltage at yellow w/blk stripe in either key on/off position. Checked radio by connecting directly to battery and radio works fine. Could this be a ground problem? Or maybe a fused link somewhere? Anyone familiar with old Ford wiring?

I assume you checked the 15A radio fuse (number 11 in the interior fuse panel) and also the 60A fuse under the hood that feeds all interior electronics?

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Well, I checked the 15A fuse, which is fine. I'm not sure where the 60A fuse would be, possibly near the battery? But, if that was blown, wouldn't all of the eletrical items be out? I still have functioning wipers, key left in lock buzzer, horn, etc. Just not the radio accessory function. I just changed out the ground wire to the battery since that was the original and there might have been a weak ground. I'm only getting 10.40 volts at the radio memory wire which is constant. I thought that was supposed to be 12 v. I read 12.5 v directly at the battery. So, maybe I have a grounding problem.

Well, you could just run new leads up to the radio... Making sure to dis-connect and insulate the "old" wires... You can run a new Constant, Accessory, and Ground lead up. Make sure to fuse the new wires just a few inches away from their attachment point. Of course, you could fix the existing problem. Yes, usually around 12.6v with key off... 14.4v with vehicle started...

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