As other posters have noted, an external antenna will probably help. But
GPS operates at normal signal levels that are below the noise threshold.
Fancy software and hardware extracts the GPS signal.

Your Nav or Com is a superhet radio using a local oscillator (small
transmitter) to beat with the incoming nav or com signal and create an
intermediate frequency. The local oscillator (LO) has harmonics that extend
into the Ghz region, raising havoc with your GPS receiver. So even with an
external antenna you might still have problems. You might try an amplified
antenna, mounted away from your nav-coms. (most remote GPS antennae contain
an amplifier that will get its power from the 396). It will boost the GPS
signal.....and if mounted far away from the navcoms, the amplified antenna
might raise the S/N ratio high enough to allow GPS operation. Go for a unit
with at least 15db of gain, but remember the longer antenna coax will drop
the signal a few db (depending on quality of the coax cable).

Regards,
John Severyn
@KLVK