Through-Glass WiFi Antenna
Parallel capacitor plates of 25 mm by 25 mm seperated by 4mm of glass with a relative permittivity of 4 would give a coupling capacitance of about 5 pF. That capacitance is in series with an antenna signal and at 2.5 GHz, would act as a blocking impedance of about 13 ohms so, it's feasible it could be used without disrupting the VSWR too much.




k is the relative permittivity of the glass. 
\$\epsilon_0\$ is the absolute permittivity of free space (8.854 pF/m)


The 13 ohms series blocking impedance could be tuned out by a small value of series inductance. I would expect it to work best when located at the edge of the window (and close to the car body) because the antenna type is a monopole and it needs some form of local ground plane to be most effective. In other words, using a capacitive connection requires that the base of the antenna is one plate of the capacitor.

I'm not going to rule out coils that can couple via the glass window but, at 2.5 GHz, these might be starting to become lossier than a capacitive coupler.