From your description I suspect that the frame has a tube brazed within the top tube, through which the housing is threaded. Water can wick into the gap between the housing and tube causing rust and binding it together. Usually I can free these by threading an inner wire then pulling hard, alternating direction until I get some movement.

But if it's really stuck, I'd use some penetrating oil first, not WD-40 but something like Liquid Wrench or Kroil. Besides oil these have chemistry designed to attack the rust itself and so should provide some relief.

If that fails I'd try warming it with a hot air gun - one made for removing paint, a hair dryer isn't hot enough. Being careful not to damage the frames paint I'd warm the tube as hot as safely possible which should soften the housings plastic outer cover.

Last resort would be to carefully use pliers to grab the end of the housing's steel coil and pull it hard enough to hyper-extend it which will reduce the diameter and lett you pull it out as a stretched wire. Be careful not to snap it. Once the wire is out then I'd use a length of bare housing chucked in a power drill as a plumber's snake to clean out the plastic.

Once you get this cleared, use plenty of stiff grease or silicone lube on the next cable to prevent a repeat.