tail light or back up lens

I have a 1970 xr7 cougar project. I am looking for one light cover. The passenger side rear corner light it is clear.- Is that a back up light or a running light or turn signal, it is very small- looks like a chevelle corner light, and where can I purchase one, for info I have a site that deals with only mustangs and vintage cougars, called mustangs unlimited. # 888-398-9898, they have tons of stuff for cougars decals suspension upgrades, stripes , wings- I probally am gonna ad the eliminator front and rear wings- about $200., consolestuff- interior, turn signal relays. this might help some one out- looking for cougar stuff, even performance parts- Nick

That there is your back up lens.