Audiovox install on an 89 GMC Truck

Hello, I have an 89 GMC, 5.1 liter, automatic, 4x4, 2dr long bed truck. I just bought an Audiovox CCS-100. I have got all the wires hooked up except the tach. I am unable to get the system to hold at any speed. I have gone through and checked to make sure my wires are hooked up correctly and as far as I can tell they are correct. I have the VSS wire connected to the speed sensor coming off the tranny, the one on the far back by the main drive shaft. The LED on the unit pulses when I roll the truck forward, when I press the set button the LED will light up as well as when I press the RES/Accel button (which all this is correct according to the instructions). Does the tach have to be connected? If I connect the tach where do I find the wire, I have not been able to find a place to connect the wire? If I don't connect the tach do I need to ground that wire? How many PPM's do I need to set the unit to? I have spent about 20+ hrs trying to get this thing to work and no luck. Thanks"Taste death..live life!"

Oh I forgot to mention it is a K2500 model."Taste death..live life!"

Found the tach wire and have triple checked all the wiring and still no luck on getting it to work. Can you pull to much vacume? I pluged the vacume line I am using onto my Motion Pro carb balancer and it measure about 56 so lack of vacume is not the issue. I am unable to tell if when the engine is running and you press the accelerator the cable connected to the cruise is sucked in as well."Taste death..live life!"