Your vehicle does not have a VSS sensor (Town Cars stopped using one since 1998 MY)
However, the PCM generates a VSS signal for various components. It is a Gray/Black striped wire.
You will need to splice into this "VSS" circuit wire for your Nav system. It goes up to your overhead console on early production '03s, the parking aide module, the power trunk lid module, and the navigation module (if equipped)
IMO, the easiest place to tap it for YOUR install is the passenger foot well. This wire passes through an inline connector (215) on it's way to the trunk for all those modules listed above.
You will have to remove the kick panel to access it. It is the very bottom connector, the wire (again, GY/BK) is on pin 7 of this 12 pin connector.
Remember, SPLICE into - do not cut this wire off from the trunk.