In each headlight housing there's an adjustment screw for aiming the lights. This screw is accessible through a 2 1/8" hole in the inboard side of the headlight housing; this hole is neatly covered by a removable round plastic cover called a "cap plug". That's the good news.

The bad news is that those cap plugs have a nasty habit of vanishing. Some C5ers have had their cars delivered from the factory with one or both of the cap plugs missing. And others have reported that at speeds of 80mph and above the cap plugs just blow away. Without the cap plug, light leaks out of that hole at night and is really annoying to the driver. Once you notice the light leak you can't seem to look anywhere else!

Go ahead; roll up your lights and see if you have both your plugs; betting money: Nope!

Here's a picture of the cap plug position:

But all is not lost (even though your cap plug(s) may be). The part is available from your friendly Chevrolet Dealer Parts Department for $0.43 each in a $0.86 two-pack. For the '98 and '99 C5 the Chevrolet part number is #10417504; for the '97 C5 the part number is #10297460 (these are somewhat smaller in diameter and are substantially harder to find). Or, you can catch me at a C5Registry event; I always carry some '98-99 cap plugs with me!

The trick to keeping 'em once you have 'em is the installation procedure. The cap plug has an inner lip that seals the plug into the housing. You have to press quite hard all around the circumference of the plug. You can hear it "click" into place when you do it right. It takes more pressure to do it right than you might expect.

In some stubborn cases, the headlamp housing is itself at fault by being too close to the adjusting screw and failing to flex enough so that the plug snaps in well. Chevrolet knows about this and, if you whine loudly enough, they'll replace the housing(s).