93 Lumina Euro tail light problem?

I have a 93 Lumina Euro Sedan that I think has a brake light problem. I has 6 tail lights with the reverse light in the center. When I step on the brake only the 2 outer curved lights on each side light up not the other 4. All 6 do light up when the parking/headlights are on. All bulbs are fine. All 6 tail lights are supposed to light up when the brake is applied right?

If they have two elements in the bulb and a 2 contact base, yes they should function as brake lights. I can't remember what those looked like when you hit the brakes.

You know the high voltage element can be burnt out but the low still function on a bulb like that right?