98 Ford E Super duty Tioga RV. Inspection time I was checking the lights an noticed the reverse lights only come on when parking/headlights are on.
I re wired all the parking lights a while ago. The reverse lights have good ground 00.2-3 ohm from out side of bulb to frame. So it's not pulling the ground through a parking light bulb.

All other lights work normally. The reverse lights only get +12v at the socket when the transition is in reverse and the parking lights are on.

Replaced #14 fuse engine compartment. And reverse lights work normally. But marker/parking lights won't turn off once turned on!

Marker lights turn off when #14 fuse is pulled. Or "D" relay is removed. They can then be reinserted and the marker lights will remain off untill turned on from inside cab.
I swiched "C" & "D" relays. Same results.