Wiring in rear work lamp

Hi all. I am putting a rear work lamp on my landy but want to wire it up so it comes on as a reverse lamp alongside the original lamp, but also so I can switch it on via a separate switch but NOT by the switch on the back of the light. I believe I will need a relay but I have no idea how to do it. Please can somebody give me some advice.

Thanks
Pug

http://www.landyzone.co.uk/lz/f7/relay-73862.html
This might help, its how I got mine done.