Not sure how many people have Hervey's third brake light (HCSL, CHMSL, whatever you want to call it), but I recently switched out my tail light bulbs with LEDs, and decided to do the ones in that as well. My understanding is that this unit is from a Corvette C4, specifically the ZR1 from the look of the pictures in a Google search.
The standard halogen bulbs have G4-style bases, and they're pretty small. The best match I found was on SuperBrightLEDs.com, the G4-WW2-COBW. They just fit in the housing, although you might have to bend the pins out a little bit to get them installed. The warm white ones look properly red through the red lens, not pink like cooler lights might look.
The first two pictures have one LED installed, and the rest are the original halogens. The last photo is the final install with all LEDs.
IMG_2763.jpg IMG_2762.jpg IMG_2766.jpg
-- Joe