Thats a new one. Iv not attempted fitting a bluetooth module to a series1.
Obviously its always a game of costs, but upgrading to a series2 headunit would be the best option as the Bluetooth is built into the device itself. All you would need to do is run the wiring the the mic in the interior roof light.
The series2 units also allow streaming music via bluetooth (Series1 can't do this) which is also another bonus.
Next time I pull out a glove box, Ill see if I cant spot where the S1 bluetooth harness connects to on the main body loom.