Run a hot wire off the ignition (or off any hot connection) to the reverse switch.
Reverse switch to reverse lights.
Make sure reverse light are properly grounded. Voila!
On my 88", I did it slightly differently. I ran a positive wire to a relay located near the firewall, and then ran a negative wire to the reverse switch, and from the reverse switch back to the relay. The reverse switch made the connection through the negative side instead of the positive. I did this just in case the wire ever snagged on something under the truck, I wouldn't have a hot wire dangling and shorting everything out.
61 II 109" Pickup (Restomod, 350 small block, TR4050)
66 IIA 88" Station Wagon (sold)
66 IIA 109" Pickup (Restomod, 5MGE, R380)
67 IIA 109" NADA Wagon (sold)
88, 2.5TD 110 RHD non-hicap pickup
-I used to know everything there was to know about Land Rovers; then I joined the RN Bulletin Board.