Grounding chasis in the electric vehicle

[nikita]

what will happen if I ground my 12v(low voltage system) and 144v(high voltage system) to a common ground?

 

[Simon Bridge]

If it is a genuine ground - then they will both be grounded.
If not then the common "ground" rail acts as a reference voltage for both low and high voltage systems.
A lot will depend on how your car is wired though. You can probably sketch out the resulting circuit diagram.