How to find the splitter box for cable TV?
Basement, attic, crawlspace, &amp; closet (removable panel?). Attached garage?

Possibly does not exist and is a direct run.

Start by finding where the coaxial cable enters your house and try to trace it from there. 

If it enters directly behind the working jack, it's likely a direct run by the cable company who did not tie into your existing (questionable quality) wiring.

See also, Why are most of my cable/coax wires disconnected?