navman goes in to down load mode when plugged in to car charger?

You need to use a genuine Navman charger, instead of a generic one.
The connector is a Mini-USB series B connector which has five pins. What I expect is happening is that normally only Pins 1 and 4(and 5, as it is connected to Pin 4) would have voltage as these should be the power in.
Pins 2 and 3 should be data, and if any voltage is detected on these pins, the device will try to go into data transfer mode.
A large number of Mini-USB car chargers do not make this distinction.
I hope this helps.

StMarks
May 2010