Using 2 antennas in car
 

I am trying to hook up 2 FM modulated devices in my car, so I am trying
to find a clever way to do so. The problem I have is the reception is
poor because the antenna is in the back window of the car, so I was
thinking about splicing the antenna behind the radio unit to put a Y
adapter, so I could add an antenna directly behind the radio unit so I
could get good reception with the FM modulator. Does anyone know of
any issues that could arise from this approach? Thanks