Choosing a vehicle antenna and mount

Installers consistently say the antenna side decides whether a mobile radio performs. Modern Diesel in Austin puts it bluntly: "Antenna type, placement, and ground plane matter more than the radio does" (source).

Common mount types

NMO through-roof mount. A permanent hole in the roof, usually the best ground plane and the cleanest look. Installers often drill near the dome light so the coax can be fished without dropping the headliner (advice from r/amateurradio installers, thread).
Magnetic mount. No drilling, easy to remove. Fine for light use; coax has to pass through a door or window seal. A Colorado installer's MXT575 review recommends stepping up from the included mag mount to a hard-mounted NMO for range (5280 Offroad).
Lip / trunk / hatch mount. Clamps to a panel edge; no roof hole. Ground plane is smaller.
Fender, hood-channel, or bed-rail bracket. Popular on trucks and Jeeps. Often the quickest install.
Roof-rack bracket. Common on overland rigs; may need a separate ground strap.

Match the antenna to the band

An antenna is built for a frequency range. A GMRS antenna (around 462–467 MHz per the FCC) is not the same as a CB antenna (much lower frequency, physically much longer). Dual-band ham antennas cover ham bands, not necessarily GMRS. Ask your installer to confirm the antenna matches the radio, and to check SWR after mounting.

Coax tips

Route coax away from sharp edges and moving parts.
Don't coil excess coax tightly next to the radio.
Connectors should be crimped with the correct dies or soldered — ask how they terminate coax.