According to the "modeltrainforum", on the UP it was the upper light that had the "Mars" feature.
Mark in Oregon
Rusty - you mention that on the Burlington F7 the MARS light is on top - do you happen to know if UP did the same for their F and E units? I have an American Models F-7 in UP paint that was never fully 'tricked out' (I think I have all the pieces) - and I bought a Dallee version of a MARS light (which I'm not sure they offer anymore). But I wasn't sure which of the two light openings had the MARS light in it.
Short answer: The MARS light (or Gyralight) always goes on top.
I have never seen a photo or in real life of the MARS light being in the lower (door) opening on either an E or F unit. Same goes for PA's, FA's, FM C-Liners, FM Erie Builts and BLW Shark Noses. The mechanism would get in the way of the door being able to be opened all the way.
Even when Santa Fe outfitted their E6's with MARS lights, they reinstalled the headlight lower on the nose and put the MARS light in the top bezel.
The ONLY exception was the Rock Island TA, where the Rock Island cut a hole in the nose below the herald to install the light.