If it's a "factory tuner" (little black box) then perhaps it has died. Quite a few of them just seem to stop working for no obvious reason, and we've never put enough effort into discovering exactly what in them fails -- it's something inside the protective RF metal "can" on the PCB. Probably an op-amp.

That may not be the issue in your case, or it may be.

Another common failure is the 90-degree antenna connector -- you can probably just pull it out (it plugs into the tuner) to inspect it.