you can buy universal ones, some which replace and wire into the existing, and some which you just unscrew the antennae and this sticks over the existing stub in the roof. depends which route you want to go for
Depending on what you buy, a sharkfin antenna may reduce signal strength for radio, compared with the OEM stub antenna. Don't forget antennas are impedance-matched to the receiver electronics, and the physical size and layout of the antenna is important compared with wavelengths of transmissions (like 1/4, 1/8 wavelength etc). This matters mostly if you live somewhere with marginal DAB or FM signal strength... Just saying, don't buy some Chinese-special based on how it looks - it's a bit more complicated than you might think.