John... the advantage of the diplexer with passive filters is you never have to remember to manually switch antennas, and both radios will receive equally well when not transmitting... with a mechanical switch you'd only have receive capability on whichever radio was connected to the antenna at the time. I have a diplexer connected to my FT-847 (separate antenna jacks for VHF and UHF) to a dual band antenna on the roof of my house; works great!
As for glass mount antennae... I have a Larsen KG-2/70 that I used very successfully on my old '85 VW Golf, on the back window, after cutting away the window tint and straddling the defroster wire... but when I traded that car on a 2000 VW Beetle, no joy as there was metallic content in the newer car's glass that wouldn't allow the inside/outside coupling to work properly.
That said... I still have that antenna and I'll give it to ya if you want to mess with it. PM me if interested.