Traffic Light Thought - How tell them, but not cause a heart attack?

It was after sundown and the dark VW Golf ahead was quite hard to see. It had brake, indicator and headlights that worked but there was no sign of normal taillights.

After following for a few kilometres finally a red light gave me a chance to...

Well, I thought a light tap on the window would announce my presence as it was dark, the lights were about to change and time didn't allow a formal introductions, but the panicked "My God I'm going to be car-jacked" (or worse) look probably meant that wasn't the best idea

...tell the driver she had no taillights. Sorry 'bout that!