Greetings,
Just put a autometer tach in mine. Mounted it under the top lip of the dash using the left most screw hole and longer screw - ran the wires through the dash vent. I used the factory tach harness to supply 12V operation power and dash lights. Unfortunately I could not use the tach signal wire as it only provided one pulse per rev and the tach needs several. Had to use the clamp on inductive pick up that came with the tach. It works well. The factory tach wiring harness on my 92 hangs right above the e-brake mechanism and is black in color. It has four male leads inside the plug. On mine, the two center leads provided the power and I just plugged into them with a standard female plug - used the ones with the plastic covers so they did not touch anything else - the factory male terminals are very narrow. Test your wires with a volt meter to see which is which - the color code in my service manual did not match the wires in my truck.
Hope that helps