The idea of using a Y-cable works. I have used that before.
BUT, depending on what hardware you use for the sniffer it only works on one pin (7).
I used a small breakout box between diagnostic socket and scanner. That way you can listen to any pin you want to. Useful when recording the communication for systems like Abs, Airbag, etc.
You do have to convert the signal from the car to something useful to a PC - you could do that using a simple KKL-interface.
I didn't use an obd-tool for listening. That doesn't give enough information. I used a program called COMWATCH that records data and timing information.
Take a look here: Universal Communication Analyzer<br> <font color="red">Discontinued Product</font><br>
Of course there are many other recording tools.