Split the case and DO NOT remove the motor and brushes and remove the remaining cable and hook.  Take a moment and remember how the hook is engaged to the drum so you know how to connect the new cable when the time comes.
 

Now take a Vise Grip and clamp it to the solid rod section of the mast and remove the tip that is screwed on that solid rod.  Some rare versions, the tip will not screw off, if you have this situation the you will need to cut the tip off.  The new cable will have the ability to have a new screw on tip to be installed so don't worry about cutting it off.
 

Now pull the remaining cable out of the bottom of the mast.
 

Now insert the new cable into the bottom of the mast and screw on the tip.  Make sure that the cable move smoothly in the center of the mast as it extends.  If it doesn't remove the cable and file around the connector that joins the plastic cable to the solid rod of that cable and take off any high points that may be on it from the crimping process of the cable to the rod from manufacturing the part.  Then install it again and make sure it move smoothly.
 

Now with the mast extended in the storage tube, connect the hook into the storage drum. Position the drum with the fish hook engaged in the drum so the storage tube and cable will be able to put the case sections back together and be able to install the CLIPS.  Make sure that the cable is positioned so that it will not be pinched by joining the cases together.  All you need is the clips to keep the cases together.  If you have a unit that has a bracket that holds the storage tube to the case then you will need to use some stainless screws at those connection points where there was a rivet.  But the remaining removed rivets don't need to be replaced.
 

Now take the power unit to the car and with the key and radio off, plug in the power unit to the car and it should run down all the way, if so then turn the radio on and it should run all the way up, if it does so you are done other than installing it bask into your vehicle.