First off, the techs at Meineke know absolutely zip about the electronics in these cars and regardless of what some tech there said, you're not going to grab any speed information off the harness going to the cluster because it is transmitted via CAN. Your aftermarket speedometer will need a pulsed output (VSS) from body control module. I've attached a picture below for your reference to keep you from really messing up the wiring in your car but I do not condone what you're doing and I accept no liability should anything happen as a result of the use this image or the information it contains therein. (You should see that I'm protecting my аss here so what you're going to be digging into has the potential to harm or kill yourself and other if done incorrectly so don't take this warning lightly)