Something

dcmd_regplate(playerid, const params[])
{
	new vehicleid;
	vehicleid = GetPlayerVehicleID(playerid);
	if(!IsPlayerInAnyVehicle(playerid))
	{
		ErrorMsg(playerid,"You must be in a vehicle to change the plates!");
	}
	else if(!strlen(params))
	{
		UsageMsg(playerid,"USAGE: /plate [REG-ID]");
	}
	else
	{
		SetVehicleNumberPlate(vehicleid, params);
	}
	return 1;
}