Park the car outside
Disconnect battery, wait 5 min
Reconnect battery
Switch on ignition to pos1 so nav system is running
Switch to service mode
Observe GPS status. You should get almanac in about 5min once you have a few satellites locked in

Note that when the nav unit fires up the first time it calibrates the gyro, so don't rock/bump the car for first 15min after you plug in the battery

Jochen