I was moving this problem many years ago: operating a VHF-Marine Radio and an Car Radio on just one antenna on the mast top.
I thought about a manual splitter too: Only problem - what happens to your VHF-Marine Radio if you forget to turn the switch into the correct position and the antenna is not connected?
The manual of the VHF gives the warning to not
operate (transmit) without the antenna connected - some important parts will be destroyed immediately...
So I installed a permanent splitter, connecting both VHF and car radio to the antenna.
Here the next pboblem arose: What happens, if you transmit with, say 25W wile the car radio is running?
You will destroy the signal amlifier of you car radio instantly because 25W are way too much to be amplified..
I installed an either/or power
swith between the VHF and the car radio, so it is never possible to operate both radios at the same time.
I installed the power swith right next to the VHF and so instead of swithing the antenna I swith the power to the equipment I want to use. And it is foolproof: you cannot destroy the VHF (because of the lacking antenna) or the car radio (because of a too strong radio signal).
I know, it is not a standard installation of the VHF as it has to be on commercial ships, but this arrangement has worked for many years now without any problems!
Only problem now: I cannot listen to stereo music wile sailing and using the VHF. But we never missed loud musik on the water.
Wintertime is time of improving the ship, standing high and dry in the shed!