Yes you will need the
entire circuit, the voltage levels on the BUS are more than 5 volts so the glue circuitry takes care of shifting levels and is required.
Depending on what your time is worth it might be cheaper for you to just buy a ready made OBD II scantool. This might sound selfserving, but by the time you obtain all the parts and verify debug the circuit you will end up spending more than the $ 85 that it would cost to get a unit that reads all the protocols. (
www.obdpros.com)
You do not have to plug into the bus at the radio itself. You can directly hook up to the OBDII connector since the J1850 bus is brought out to the DLC.
Paul
www.obdpros.com