another take on modified antennas ...

I have a magmount dual-bander on the roof of my mini van. It terminates in
a BNC for use on an HT.

The stock AM/FM car radio antenna went bad with an open shield and it was
picking up lots of noise. I pulled the radio far enough out of the dash to
unplug the bad antenna. I plugged in a Moto-to-BNC adapter and dropped a
short piece a piece of RG-58 down to where I could mate it with the mag
mount's cable connector. Works great. Quick disconnect BNC lets me go back
and forth as necessary.

BONUS: For our regular trips across the desert, I have an extra-long whip
element that I substitute in the magmount base. I lose the match at 440 but
it still works like a champ for 2M or AM/FM, my choice.

I have not played with diplexing as swapping connectors seems too easy.
Yeah, a switch maybe ... someday.