When retrofitting a satellite navigation head unit to the car you can either install the factory GPS antenna on the roof which is a fairly large job or install the GPS aerial (antenna) in the area above the head unit where it receives a good view of the sky and minimal interference.
Remove the dash vent (or tray on non Climatronic units) by following this guide.
If you need to remove the head unit remove the trim around the head unit using this guide.
Remove the head unit by unscrewing the 4 screws and carefully sliding it out, watch out when the unit is released as it could drop and scratch the climatronic.
Using double sided tape stick the GPS aerial in the location shown and press down, wedge some foam on top and tape the GPS wires down to prevent them interfering with the diffuser vent (for cars fitted with climatronic). The photos show two GPS aerials installed side by side and there is room for two more.