where is the block to hook up jumpers to read codes manually?

:banghead:
I want to manually read the codes in my computer, its a 93 s10 blazer 4.3L z code 2wd 5speed, i read you can manually read the computer codes by hooking a jumper wire to the a and b terminals in the test block, the book says it's under the dash on the passenger side but it's not there, the only thing there is the computer. I read you jump with the ignition off, then turn it to run, the check engine light will blink 3 times the in a succession of blinks to indicate stored codes with a pause in between, sounds simple and i have a list of all the codes the blink will indicate but can't find where to put the jumper, anyone know where its at on my truck? thanks for looking.

usually under the dash on the drivers side