For a car navigation system design, you need to know both analog and digital design.
The analog part is to build the GPS subsystem to get the GPS signals and down convert it. Then you need to know digital design to interface the digitized GPS signal to the Base band processor and the navigation display/map application.
Analog:
The analog part can be simpler if you use a dedicate GPS receiver which can give out NMEA sequences from the received L1 GPS signal. Still you will need to design/wire the Rx front end for GPS. So you can concentrate on antenna design, LNA and filter design for 1575.42MHz with at least a 2MHz bandwidth.
Digital:
This is get the NMEA sentences from the GPS output and convert it to some useful data... Basically you will need to decode the Longitude and Latitude information and display it on a map... Using a microprocessor/controller should be the easiest way... So you will need to know a little bit of programming. But if you intend to design the full system, you will need a lot more than that... you will need to interface an LCD and add software to view the map, and display your position in the map using the decoded Lat/Long information.