Shortening the length of an antenna is a tricky thing. The factory length is calculated to ensure the length is a certain percentage of a wave to efficiently captured it. Shortening the antenna length means the that windings and such have to be perfectly sized to maintain that ratio. When closer to radio transmitters there is more forgiveness due to the higher power reaching the (maybe) less than ideal short antenna. But as distance increases, like if you are trying to get NYC, Connecticut, etc. stations, the reception is going to be suboptimal. Unless the antenna you buy has the exact wire measurements listed in the specs sheet it is going to be difficult to ensure your purchase matches the calculations you make. Research how to shorten an antenna, you will see a formula. You should plug in the number of the station you mostly access (average input is 100 for the middle of the fm dial but you may want to use, say, something a bit higher if you listen to 106.7 WLTW more). The visual way to understand is look at your current antenna, there is a wrap around it and the helps with reception (and maybe reduce wind noise). Your rubber antenna needs the same length inside the rubber to work near as well as your current one. Any deviation in wire length changes reception characteristics.
Do this may be why one antenna works so well for one person and not another. The first person may listen to a station where the wire length matches the frequency they listen to (example 103.7) where second person is more at 93.7 in the dial.
Long story short, you can't just put any antenna on a vehicle, it is complex. Good luck.
