Two possibilities;

Wrong wire
Bad Test.

Confirm the polarity of your test probes.

I would not bother running a ground to the battery. Any metal surface on the body will give ground. I like the top stud that the cover to the factory amp bolts onto.

Tests would be as you described

Positive probe on wire
Negative probe on ground
make sure meter is set for DC volts NOT Ohms reading...

Car Off
No Volts

Key On engine not running (neutral with brake on)
no volts

Key On engine not running (reverse with brake on)
>11 volts

If you still get the same test results you got before (power no matter position of transmission) clean up that wire and tape it off. Instead use the lead behind the side panel in the back of the car that goes directly to the reverse lamp in the diagram I showed previously.

You should be able to easily test it the same way and get the expected results cited above. You can easily see the reverse lamp lit or dark and only have power when it's lit.

As I said, I decided against using the wire in the floor up front because I didn't want to unbundle the harness and I had an easy short run from the reverse lamp to my control box. For you it would just be a single 18g lead from the reverse lamp lead under the carpet and side trim (probably can sneak it in there without removing anything) all the way up to your unit.

 

When it is in the "run" position, it's getting power.. so you can turn the radio on when it's in "run"... that seems like a good thing to me

 

If you were looking for power for the head unit sure it would be a good thing. But he's looking for the signal to tell the GPS unit when the car is put in reverse. The lead he is looking at is always "hot" so it won't work for detecting reverse.

 

Thanks ppro. I'll take your advise and run a wire from the back-up light.