No, damage would not be covered by the Nissan warranty. Can tell you the headliner has to be pulled in that area in order to do the repair/replace the antenna base itself. Parts, base and antenna and repair/install total will run you about $275+- at a dealership is my guess.

If the antenna mast metal screw in male fitting has just snapped off and there is no damage to the base then you can probably get the broken mast metal male end out of the base's female fitting and then just replace the damaged antenna mast. Nissan antenna mast, new costs about $26 on eBAY.

You do say though, the part that the antenna screws into is damaged. Is the metal female end in the antenna base still connected to wires and just the plastic base itself is cracked/broken? If so you could just buy a functioning aftermarket shark fin, install same and hide the damaged antenna base. Install of the shark fin is easy, easy and costs about $70. Look here --> VG Shark Fin Antenna - What Is It? I recently contacted the vendor and he still is offering the fin and can provide exact color matching for the 2011 and older Muranos. For you, with a damaged base, you would still have to have an undamaged, wire connected female fitting (cracked/mangled base is ok as it will be hidden/covered) and my guess is you do... See pic below.

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It snapped it completely off. I can't even figure out where the wires attached.

 

Look at the antenna base on the roof. Is the plastic base damaged or cracked? The base itself has a female fitting with hidden wires that the antenna mast/whip screws into. There may be a small brass adapter between the mast and the base. In your pic it looks like you have an adapter with the male tip broken off. If you look at the base on the roof, I'm quite sure what you will see is a broken tip still remains screwed into the female end there. You would need to extract that broken tip out of the female fitting. Drilling a small hole into the remaining broken tip in the base and then using an extractor, the tip then can be removed. After this all you will need to do is to buy a new mast or the shark fin if you want and install. For someone with the necessary tools and the inkling this can be an easy task.

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