This is the step that gets complicated, especially on late-model vehicles. The trick is figuring out aspects of the management system such as vehicle speed, TPS, mass airflow, and upstream and downstream O2 sensors. Then there's the auto shift relay, injector drivers, and a number of other electronically controlled components to sort out. Lastly, you must determine what not to use, such as electronic door locks, the ambient temperature sensor, and keyless entry. We are still working on the left harness plug-in, but luckily the harness was intact, so we just needed to plug and play. Make sure the OBD-I or -II plug-in is also included for engine diagnostic checks.