Ive had decent results from the FM radio splitter setup, before I completely removed the FM radio and installed a TK-880 GMRS radio and a BCT-7 where the enormus GM brand FM radio was housed. Since then, Ive just used a motorola to BNC adapter. While comparing the FM music radio antenna to the NMO mount 5/8ths wave 462 mhz antenna, ive noticed about half the range using the FM antenna. For VHF stations, Ive found that the difference is about the same. For VHF low, it doesnt work as well as a trimmed CB antenna.
Overall the type of FM antenna on your car will vary from other models. Mine is a 1992 chevy, and the FM antenna is rather poor compared to a NMO mount antenna tuned to the right length. I plan to get a similar GM antenna, cut the old one for 153 MHz, and see how much things improve. I wonder if the factory coax is any good? Its possible that it is losing quite a bit of signal, or maybee its even dry rotted or pinched.
Overall the FM antenna coupler is a good compromise when you dont want a bunch of extra antennas.
Myself Im always interested in the low profile set ups used by local and federal law enf. Some of the stuff sounds beyond belief, but of course its hard to come by, with the exception of the paragraph in every edition of police call, that mentions a FCC capable car with DC to Daylight monitoring capability, all 100% sterile on the outside (Rat Shack... LIE? Noo!)