I would try to thread half a dozen VHF rated ferrite toroids onto each of
the gps and tx antenna coaxes, and secure them as close to the respective
devices (antenna, gps receiver) as possible. I would also try to use a
small inline transmatch in the antenna lead when installed in the car and
see what happens. I suppose that the antenna tries to use the car roof as
ground and you get respectable RF currents in it. You can also try to
insulate the gps antenna from the roof at the mounting point (so it is
grounded only remotely) in the hope that this will get it out of the
ground current carrying cross section of the roof. Last, as someone else
said, do not receive and transmit at the same time.

Peter

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