All i read was to put it on a water pan which makes no sense. Put what on a water pan? Pan is too small to hold a grill and i worry it could cause coals to stop burning.
Probably they mean one should put the grill on a large receptacle full of water, to avoid damaging the car like poor
@hersheybar did. If the pan I have is too small, I'd find something larger.
Removing all the flammable bits (seats, carpet, etc) from the area if the car where I intend to place the charcoal would be another option for people who don't mind strenuous activity.