Do you know how the TomTom sends audio through bluetooth? If it just does serial (RS232) over bluetooth, that would be the easiest to work with, but it probably doesn't do that... The first thing you need to figure out is what it does do, and then we can figure out where to go from there.

Are you able to connect the TomTom to a computer that has bluetooth?

You're going to need to hook up a bluetooth module to a microcontroller and then you'll need to program the microcontroller to be able to control the bluetooth module. This is going to be the hardest part, and it is dependent on how the TomTom works with bluetooth.