Search Amazon.com for "coax Toner". A toner is a device that comes with 2 parts. An end that generates a tone down a wire when connected to it (some kits come with multiple of this end to easily tone multiple wires at one time), and the other end (the wand) that makes a lot of noise/flashy lights when it is touched to the other end of the same wire. There are a number of choices <$20 on Amazon.
I'd go with rwbradley, buy the tone gen and find it that way. Mine even has a RJ11 plug on it and I can trace phone cables and a continuity checker if shorts may be your problem. Once again, a cheapy from