The Towbook integration with WebFleet Solutions (a Bridgestone Company) provides some great features.
Go to "Settings" → "Company" → "GPS Integration" → Select GPS Provider drop down → Enter Credentials.
Next, link your Towbook Truck to your GPS Unit.
Once all the Towbook Trucks are linked to the GPS Units, move on to linking your drivers.
Go to "Settings" → "Drivers" → Select & Modify a driver from the list.
Link the Towbook Driver to the account created with the GPS Provider (WebFleet Solutions)
Once you link all of the Drivers, last item is the Driver Assignments. Go to "Settings" → "Trucks" → "Driver Assignments".
Here is where the Driver is associated with the Truck/GPS Unit.