You can use the factory navigation system by installing a switch on the VSS (Vehicle Speed Sensor) wire going into the back of the headunit.* You could simply disconnect it, but you will lose some accuracy doing this.* There are a few writeups on the internet showing you how to do this.*
Here is one on CAFCNA.* If you search there for VSS, you will find about a half-dozen other writeups on it, with lots of information.