A appreciate that this maybe slightly OT, but I am sure someone here can
help.

I have a GPS reciever that also recieved RDS broadcasts (i.e on a standard
FM frequency)

The current GPS unit uses a single wire with a phono type plug as an
antenna.

As an alternative there is a "Y Adapter" avaliable that plugs inbetween the
radio and aerial which gives the GPS unit the use of the main car antenna
(improving reception). See links below. The official splitter costs around
35 Euro, but I am sure that there is not much to it and a DIY solution would
work.

Can any recommend the best way to go about this? to provide the GPS unit
with a decent aerial feed?

http://www.gns-gmbh.com/en/ Quick Find: 4037735101388
http://www.navigon.com/site/int/en/s.../universal/102