Take the cable out and hang it up to oil.
Put a money bag on the top and tape it around the tip of the cable router. Fill the bag a bit with 3 in 1 oil and hang over a dish or something. Sit there and pull the bottom bit of the cable in and out for a few mins. Leave to soak and repeat in an hour.
Brought my original one back to life in a hour doing this last night after the apparently cheap one on the bike snapped. Had been sitting in the garden for the past year and could barley move it. Brand new now
Oh yeah, clean and grease your worm drive