[Calgon]

pawn Код:

public OnPlayerCommandText(playerid, cmdtext[])
{
    if (strcmp("/mycommand", cmdtext, true, 10) == 0)
    {
        if(!IsPlayerInAnyVehicle(playerid)) return SendClientMessage(playerid, -1, "Your car has not been tuned as you're not in one."); 
        TuneVehicleRandomly(GetPlayerVehicleID(playerid));
        return SendClientMessage(playerid,-1,"Your car has been tuned.");
    }
    return 0;
}

Using GetPlayerVehicleID() informs the script of the vehicle ID you want. It selects the vehicle ID the player is currently in who executes the command.

I've added a check to ensure the player is even in a vehicle too, and removed the pointless return.