89 Saab 900 Error code readout

My 89 Saab 900 is acting up and I need to do a read-out of the fault codes. I need someone to describe the proper hookup for wiring of the readout switch to the four wire connector under the rear seat. I've got four wires, red, black, blue/white, and gray/white. I believe once I get connected I understand the signalling handshaking. I just need to get wired up.

thanks,

jim