I think what you are looking for is
cc:acceleratorOn(true)
Take a look at:
https://projectzomboid.com/modding/zombie/core/physics/CarController.html
However if you are having trouble with getting the CarController, try and get the vehicle using :getVehicle(), returning the BaseVehicle which you can use :getController() on to get the CarController from the player.
Take a look at:
https://projectzomboid.com/modding/zombie/vehicles/BaseVehicle.html#getController()
I'm new to all of this so, sorry if this doesn't help.
