I found a wiring diagram via google, as well.
As AZPete says, each light does in fact have a different color code on the wire that supplies power while braking. If you could scrounge a 3-wire plug for the inner light that would actually be pretty easy to splice in.
2005 Mustang GT Premium
CMS Stage 1 cams, C&L Racer, Lito tune, CMDP, SR UDP's, JBA ceramic LT's & o/r shorty H, Magnaflow resonators, Ford Racing AB, Steeda Tri-Ax, BMR springs, Koni STR.T, BMR lowers & brackets, J&M upper & mount, J&M AL panhard bar, DIY brace, smoked markers/LED tail lights
2009 Ranger XL SCab- 4.0, auto, 4x4