I have the OEM Shark Fin antenna from Japan. It is Starfire Pearl. I slightly modded the metal plate with a dremel tool by cutting out a hole large enough to that the cable ends could slide through.
To install, I simply removed the old antenna, opened it up and unscrewed the 3 antenna elements. I covered them in electrical tape and was able to fit them into the Japanese antenna (what I believe to be the FM/AM antenna I slid vertically into the fin, the remaining 2 antennas fit flat on the bottom). I then reconnected the antenna to the car (plug and play - no cutting was required) and had a regular sized shark fin antenna.
I noticed no sound quality difference with the tweaked antenna orientations - sounded great (FM, AM, XM and NAV work without issue).
I am asking $200 and an additional $10 for shipping.
Shark Fin Antenna - Imgur