Thanks for the reply Odia and thanks for the info page.

I deffo have the transistors in the right way, I was just clutching at straws, I've tried just about everything I can think of.

I think that jochen is right, for some reason the the transmit circuit is not working, why I don't know, it's all soldered up OK.

Does the GND connector have to be the one from the cd interface? (the cd 3 pin plug), I have it connected to the car body.

Also when I connect the circuit up to the laptop and add 12v and GND to the circuit the navcoder software seems to freeze untill I unplug the i-bus circuit, then its back to normal.

I'm back at work tomorrow so I might see if i can get one of our lab techs to check the circuit over...

Thanks again

John