Can anyone tell me how to take apart the power antenna mast tube? I have the mast separate from the motor, but can't figure out how to replace the broken cable that attaches to the antenna mast?

1. You can't separate the mast from the cable.
2. Once you replace the mast (www.acdelco.com, search for power antenna), you will have to disassemble the motor to get the cable in there. Your motor may be the kind that you can't take apart.
3. If your motor cannot be disassembled, look for one from either a late-80's or pre-92 W-body (Grand Prix, Lumina/Beretta, Regal, Cutlass Supreme). The motor housing will have three brass screws holding it together. Remove the screws, remove the guts, and put the new mast in.

The mast has to be removable, look at the following link:

http://www.moderndaymuscle.com/inc/sdetail/796