what would the delta_V_gnd and delta_V_in be ?
They depend on all sorts of factors, such as the capacitance of the vehicle body to earth, the capacitance of the input rail to earth.
In most cases, the capacitance of the input rail to earth will be just about zero, as the input rail will only have capacitance to the vehicle body. In that case, the voltage between the vehicle body will and ground not change significantly during a transient.
However, the voltage of the vehicle body to ground doesn't make any difference to the effect of any voltage within the car. Electronic modules on the car are not connected to the earth, only to the vehicle body, so it just doesn't matter. All that matters to them are the voltages between the terminals of the module.
If I measure the voltage of a vehicle body to ground, the reading will be meaningless and affected by lots of things. It's very common to pick up the ac mains frequency when anywhere near normal mains supplies.
The situation with the 12 V batteries is similar. Most of the time, the -ve is at the same voltage as the car body, becasue there is a great big wire keeping them at the same voltage, and that leaves the +ve terminal at 12 V. However, when that wire is not there, then the voltage on the battery terminals is different. Some common situations are:-
Body disconnected from battery -ve. Battery +ve will normally be at body voltage, battery -ve will be at -12V
The second 12 V battery in a 24 V system, where two 12 V batteries are in series. Second battery -ve is at 12 V, and the second battery +ve is at 24 V.
Positive earth vehicles. Battery +ve is at body voltage, battery -ve is at -12 V.
Battery disconnected both ends. Both voltages unknown and unimportant, but there will be 12 V difference between them.
The point is that all voltages have to be referenced to something, and if there is nothing to reference to, there cannot be a complete circuit, so there can be no electrical effect.