I'd have said the incandescant bulb since it's cheap and was invented in the 1970's, plus I think modern cars probably only use around 6 or 7 these days.

But it already mentions the light bulb.

I thought another possibility would be something to do with gears since most cars use 6-7 gears but I can't think of a cheap device used here.

It must be something really fundemental to apply to all cars everywhere.