Random radio power wires switch polarity

I'm not using the multimeter backwards, if I was then when I switch the needles the meter dial wouldn't go the wrong way. I know wires aren't supposed to switch polarities but they did, and apparently a fuse unrelated to the radio causes this to happen.

Nonsense. Car stereos operate on 12V DC. There is no switching polarity possible.

That's what I always thought but we've both been proven wrong.I even tried a different multimeter and came up with the same results.I don't know how it's possible or exactly why but it happened.

2000 Ford Mustang Stereo Wiring Information Radio Constant 12V+ Wire: Light Green/Purple Radio Switched 12V+ Wire: Black/Pink Radio Ground Wire: Black Radio Illumination Dimmer Wire: Light Blue/Red Radio Antenna Trigger Wire: Orange/Light Blue Left Front Speaker Wire (+): Orange/Light Green Left Front Speaker Wire (-): Light Blue/White Right Front Speaker Wire (+): White/Light Green Right Front Speaker Wire (-): Green/Orange Left Rear Speaker Wire (+): Gray/Light Blue Left Rear Speaker Wire (-): Tan/Yellow Right Rear Speaker Wire (+): Orange/Red Right Rear Speaker Wire (-): Brown/Pink

Yes I know I have have the repair book.it doesn't matter how many times you say it can't happen it happened.all I know is it was somehow caused by a fuse that controls the wipers.once I found that guy and replaced it everything was happy.