My stereo happens to have a cable (F connector) input in addition to antenna screws. YMMV. If yours has one, use it. If yours doesn’t have one, then a simple adapter available at radio shack, k-mart, walmart, etc. will do the trick.
One of two things will happen. Either you will get really really good reception, or you’ll get really really bad reception. TV cable, even though it is shielded, often picks up FM radio signals fairly well. In many places the cable company will intentionally broadcast FM radio stations down the cable, so you may find a wide variety of stations available.
However, since the FM radio band happens to be close to one of the TV broadcast channels (channel 7, IIRC) some cable companies intentionally filter off the FM radio band (and all the other non-TV frequency allocations around it) to prevent interference to the video signals. If this is the case with your cable company then you won’t get squat.