at the top(but under includes):

forward Update(playerid);

the command:

if(strcmp(cmdtext,"/nos",true)==0) { if(IsPlayerInInvalidNosVehicle(playerid,GetPlayerVehicleID(playerid))) {
SendClientMessage(playerid,COLOR_WHITE,"SERVER: You cannot add nos to this vehicle."); return 1;}
if(IsPlayerInAnyVehicle(playerid)) { if(GetPlayerState(playerid) == 2) { new vid = GetPlayerVehicleID(playerid);
AddVehicleComponent(vid, 1010); Timers[playerid] = SetTimerEx("Update", 20000, true, "i", playerid);
SendClientMessage(playerid,COLOR_WHITE,"SERVER: Your current vehicle now has Unlimited nitros."); return 1; } else {
SendClientMessage(playerid,COLOR_WHITE,"SERVER: You must be the driver to add nos."); return 1; } } else {
SendClientMessage(playerid,COLOR_WHITE,"SERVER: You must be in a vehicle to use this command."); return 1;}}

anywhere:

public Update(playerid)
{
	if(!IsPlayerConnected(playerid) || !IsPlayerInAnyVehicle(playerid))
        return KillTimer(Timers[playerid]);
    if(IsPlayerInInvalidNosVehicle(playerid,GetPlayerVehicleID(playerid)))
    {
    //do nothing
	}
	else {
	new vid = GetPlayerVehicleID(playerid);
	AddVehicleComponent(vid, 1010);
	}
	return 1;
}