Design of Wiring Harness Tester for Automobiles
A few other things to consider. Is this a 12 vdc system (the most common voltage on autos) or 24 or even 48 (commercial and military)? As a general rule of thumb you want to test at two to three times your rated voltage. So 24 to 36 VDC. for a standard 12 volt system. This helps find weak spots in the insulation. 

Next, is this strictly power and signal wiring? Most autos today use data buses like CAN. In this case you need to send signals, not just voltage. You also need to look for cross talk. 

You need to pay special attention to any shields. Ignition wiring must be adequately shielded to prevent EMI. 

There needs to be a load on the system and not just testing for voltage. A high resistance connection will show a good voltage as long as current is low. You could test for resistance instead of voltage, but I have seen a few cases where even a circuit that shows very low resistance cannot conduct enough amperage. 

Finally you should subject the harness to flexing and even thermal effects to simulate what it will experience in use.

I realize this an extensive list for testing. Some will argue it is not needed, but a bad wire harness on a car or truck will lead to expensive down time and repairs. I know I had the misfortune of having such a car a couple of decades ago. I still will not buy that brand of vehicle.