was this standard or did somebody add this? it looks factory. has anybody changed the bulbs? lookls like i have to take a part the whole b pillar trim to get at it

No 3rd brake lights on Corvettes until 1986. Yours has been added....but it looks nice.

According to the pictures I found on the internet, that is the factory CHMSL.  You might see if you can pop out/remove the red lens to gain access to the bulbs.  You can also find things like that on YouTube sometimes.  

Corvette Black Book agrees with Adam.

ok yes, i have now deduced that it was an 86-90 c4 unit. damned thing looked factory and i thought it may have been one of the early cars to start adopting that feature in the 80s. bummer is , i cant see how the red lens pops out. there is a body colored bezel that seems to lock it in and i dont want to pry on it. i found some on ebay and it looks like an odd lens setup. 

Hi, It may sound a bit laborious, but check to see how it's mounted on the body. It may be possible to replace the bulb from the bottom. Remove it from the car.  OR, if is came from a vette originally, there may be a thread as to how to change the bulb from the year it was O.E.M. 

ok so i ended up having to pull the trim down and saw the bolts on the inside of the b pillar. the unit came off pretty easy, then had to take apart the actual bulb housing. all the bulbs looked ok so i traced the power wire back to make sure there wasnt a break. turns out the wire was just shoved in the positive side of the brake pedal switch and had pulled out. i used a suitcase connector to reattach it and now the light works.  i must say , that housing is made to last.. its diecast and the bulb housing is earthquake proof. i just wish they has a face that came off so you could just access the 4 bulbs in it!  so a sore back and one screw lost to the carpet monster and i got away clean