If the antenna extends and retracts, then the inner cable is still OK. You shouldnt need to disassemble it. Look at the new antenna mast. There is a sleeve that fits tightly in the antenna tube. This often gets corroded and seizes in the tube. Turn the antenna on, but hold it so it doesnt fully extend. You will just feel the internal clutch slipping. Then carefully yank the antenna mast up a few times to try and remove it from the antenna tube. If the sleeve is still stuck, you might try and spray it with penetrating oil. Worst case, you pull the antenna mast out, but the sleeve it still stuck in the tube. A small needle nose plier may be able to work the sleeve out, or last resort, remove the whole antenna motor, remove the antenna tube, and use a long rod from the opposite end to knock the sleeve out.