I am going to use the same car antenna for AM/FM and 2M. This means I
need a diplexer that transitions somewhere between 108 and 144 MHz. I
haven't seen something like this offered commercially. Has anyone seen
a product like this, or failing that, a design I could duplicate? The
antenna will be a 1/4 wave on 2M, and would also be used as a (not
quite ideal) AM/FM antenna.

73 Dave WB0GAZ