This may go against the concept of manually wiring up vehicles and being involved in the process, but converting existing cars to physics cars is pretty miserable as you'll most likely have to ghost inside of the car constantly to rewire things. The Reselect Last Port button can help sometimes with this, but if you accidentally select a wheel's port you have to ghost in and reselect the engine again.
A dream fix for this would be a UI visual representation of all the components on the vehicle you could then click and drag between to connect (I assume the upcoming microchips will have something similar to this) but in the meantime a hotkey or something to automatically set up the vehicle's wiring (connecting all wheels and maybe even the last selected seat to the engine) would work fine in 99% of all cases and would be great for new players who most likely want to create a vehicle as quickly as possible.