Different cars have different CAN buses available. There are often multiple different buses. Some cars make multiple of these buses available via the OBD2 port and some don't. It's becoming more common with newer vehicles to have gateways in place in front of the OBD2 port that act as a firewall.
That being said, you can look up the details for your vehicle and find where other tap points might be (eg. behind your headunit or in other wiring locations).
Based on Gary's reply to <a href="https://plus.google.com/103292489750574578730/posts/c3kR5GBVFCc">this Google+ post</a>, it looks like the default pins are the High Speed Bus on pins 6/14 on the OBDII connector.