Jerry Phillips decided to add:

Had to completely dismantle mine to remove old grease and grit that had got
in with it - I used a general "cheapo" thick motor grease to literally fill
the whole mechanism - no problems since, just a build up every now and then
on the outer cap that needs wiping off...

check it hasnt bent a little as well as a good clean .Any oil means it will
retain dust and grind away .Just a small twist can ruin its sliding motion
so bend carefully thumb and fingers and check from every angle till it is
straight .

You need to remove the mast and properly clean the whole gubbins initially
with a solvent to get all the old crusty crap off it before re-lubing.

With the mast down, undo the cap nut at the base of the mast, then using a
small screwdriver prise up the collar, whose shoulder is just visable after
removing the cap nut.

then with an assistant manning the radio, power on and as the mast comes up
keep pulling on it up and out from the car with the plastic drive wire
following, which will snap back into a coil as it comes out.

Clean and lightly  reoil / grease.

Refitting, with radio on feed the coil back down at the same angle as it
came out til it stops. Turn off radio and manipulate the wire back down
persuading the motor to grab it. This might need afew attempts.

Re assemble collar and cap nut and test.

If its been sticking for too long the 'cogs' on the drive wire may be
damaged and replacing the mast is the only option. In the UK they are kept
in stock at the dealer and are just under 30UKP.

Tim..