Well its got be be somewhere! Lets start at the beginning..
1. check the battery voltage...12.6V to 14V volts is good.
2. Now, move the black lead of your meter to the chassis (assuming a positive ground, if negative ground, move the red lead to the chassis), do you still read the same voltage as you did right at the battery? If so, it's not your ground cable(s).
What year is your car? Is there a schematic on line that I can access? What have you already checked?
On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"
'68 Sprite 1275
'76 Midget 1500