No way to make that antenna work well for AM/FM, the frequencies are way too different.
See http://www.sonicelectronix.com/cat_i...-antennas.html for a list of some universal antennas including a couple of shark fin models. I recently used a Metra 44-UA46 that has a 6" flexible whip. The reception wasn't that great even with the built-in amplifier, so I swapped out the whip for an 11" cheapie from Ebay and it works a lot better.
I have no idea if their shark fin models will work better, so good luck!