What GPS are you using?
You can use your existing digital map as raw data to be edited by MapEdit (www.geopainting.com) and then compiled by cGPSmapper (www.cgpsmapper.com). Finally you can upload your own map to your gps.
If your gps is Garmin, you must have MapSource software in the package. So you can use it in your carpc for RTN (real time navigation).
May it help you...