Hello I will help you with your question,
If it is just the mast that has broken (sounds like the motor is fine) then remove the nut on top of the rubber grommet and have a helper turn on the radio while you tug on the antenna to unwind the mast and tape from the motor cavity. Note the direction of the teeth on the tape and compare the end of the tape to the end of the new tape/mast assembly. If the ends match place the tape in the tube pressing gently with the teeth pointing the same direction as the old tape and have your helper turn the radio off. Guide the tape and mast into the tube as the motor pulls. It may take a couple of cycles to get it all the way down. Install the top nut and you are finished.
Let me know if the ends of the tape do not match
The grommet is below the nut and is not removed.
You do not remove the motor or tube just the antenna mast and tape that is attached to it.
Here is a link to a video showing the procedure, it is not an Optima but it is the same procedure
Click here for link
Let me know if you can find the nut
You may be missing the nut? Is the mast there? You do NOT remove the rubber grommet in any case.
The nut is a hex with 12mm flats that is chrome.
Do you still have a mast in the tube?
I don't know what I can do unless you can take a picture of what you are seeing.
I will ask again, Is the mast in the antenna tube or is it missing? This may be why you cant find the nut it is gone.
Let me know so I can help you
That may be the problem then, the nut is not there for you to find.
The mast you got is from KIA?
Does it have a nylon tape attached to it?
Does the antenna in your car say KIA on it?
Maybe someone has tried to install another antenna assembly?
Let me know what you are seeing so I can help you
Get a mast from KIA and if it does not have the nut with it get one of those also. You should be able to see the threads on the tube that the nut screws onto if your nut is missing. The mast at KIA is about $12 last time I checked and you can order online if necessary.
Once you have the mast and tape try putting the tape in with the teeth facing forward, if it feels like it won't pull it in you may have part of the tape stuck inside. Remove the side cover of the motor assembly and remove any broken tape. Be careful when you open the assembly, the parts can fall out of alignment.
Let me know how I can help you