Kind of a smart move to put them in the backup lens since they weren't obvious and didn't attract attention as in the running/brake lights. I was told by the officer that ticketed me (more than once), that the blue/purple hue was in conflict with the blue lights that emergency vehicles use. Seemed pretty dumb, but some people are easily confused I suppose. I relocated them to the backup lens as an act of defiance and never had a problem.
I actually got more attention and crap from police for having a whip antenna with a light on the tip. I don't know how many tickets I got for that. I was able to get away with it for awhile by tinting the back of the bulb red and the front orange, mimicking the tail and parking lights. I finally stopped using it when an officer threatened to impound my car after repeatedly stopping me. He would follow me home, sit there and wait for me to leave, and then follow me wherever I went. As long as I didn't screw up, he'd leave.
