Headlight shroud adjuster access plugs, one of mine fell out while driving (realized 5 minutes later what that was that flew by my windshield, doh), will all years fit in my 98 C5.

I have read they will and I have read they will not, which is true

 

From a google search:

Headlight bezel plugs
There are 1-1/2" plugs that are located on the headlight bezels that would fall off on the older Vette's. The new style plugs, have a twist-lock to prevent them from falling out, but also requires new bezels. Only the new bezels & plugs are available now, and they are compatible with older C5’s.

 

They come in two sizes .......

 

Its two different types...a plug in which will fall back out and a turn-lock style. It depends on which surround/bezel you have.

 

there are two sizes one is 2.20 one is 2.06
one fits 97-99early the other fits late99-04

 

Prior to '99 or '00 the larger plug is required. NLA from Chevrolet and as mentioned above, their solution is to replace the surrounds which are expensive (for a piece of black plastic), in order to use the smaller twist in plug. Ecklers has reproduction "large" plugs that I put in my '99 FRC 6 months ago, and they're still there. They went in VERY tightly. My $.02.

 

I have an extra one that fits the 98, I sent you a PM

MAC5

 

Looked up the Caplug on their website and requested a free sample. They sent me one for no charge. Then use Silicon and seal the sucker to the surrounding shield. If you need to adjust remove the 3 screws. PS take out the 3 screws to insert and seal the caplug. Dealer replaced a headlight on mine so I have both kinds. Both are sealed with silicone.