Make sure the ipod firmware is at the latest issue.
You say 2.5mm jack I thought that the ipod connected to the tomtom with a 3.5mm jack one end and a docking connector at the other, are you sure you have the right cable?
The 2.5mm jack is for connecting the headphone socket on the ipod to an aux input on a car stereo.
You should have 2 cables
see here