Hooked up imported $20 back-up camera to A/V input, my Magellan 7250T-LMB

Having trouble on my 7250, which I bought on Ebay expressly for back-up camera compatibility. I pulled my hair out until I pulled out my $15 ohmmeter and "rung out" the 3.5 mm to 3 female (Red, White, Yellow) RCA adapter plug. (You need to buy this adapter (online?) to plug into the A/V input). The plug has 3 rings (for 4 conductors if needed, but the camera only sends video, and common signals). Online I found out that Magellan A/V input uses the 4th and final section from (and counting) the tip, as the video signal, and the 3rd section as the common. This is in reverse of normal (thanks, IcePilot) . So, by "ringing out" the Yellow (for video) connector I found out the A/V adapter I had bought was wired normally. I needed to reverse the input signal and common, to make it work. First I looked over the search engines for a polarity reversing adapter, and couldn't find one. So, I cut the yellow A/V cable from the receiver that came with the camera kit, soldered the internal 2 wires, (red to black, and black to red), isolated them, and shrink tubed the cable back together. Then plugged the yellow male to the A/V adapter yellow female, and voila, there was my back yard displayed on my GPS. Wonder why Magellan had the video reversed inside? Their $200 kit must be built with reversed polarity, for some reason.