Swoosh,
I am not sure I have one of the underside, but I am actually building two of them up as we speak, so I will take some shots.
I am using both LED's and incandescent warning lights depending on what they do and how they are hooked up as some applications wont allow an LED light to be used (at least the ones that I have, not sure that it applies to all LED types) So for instance, if I use just one light for the turnsignals, I have to use an incandescent since both the left and the right turnsignal leads will go through it (each hooked up to one of the leads from the warning light). However if I use a seperate left and right turnsignal light, then I can use my LED's since one of my leads from those LED's lights needs to be grounded.