[Shadow0707]

Firs you must get if player state if he is driver or passenger

pawn Code:

new pstate[128];
    switch(GetPlayerState(playerid))
    {
        case PLAYER_STATE_DRIVER: pstate = "Driver";
        case PLAYER_STATE_PASSENGER: pstate = "Passenger";
    }

Get player name and the state when sending a message

pawn Code:

new pname[MAX_PLAYER_NAME+1], msg[128];
GetPlayerName(playerid, pname, sizeof pname);

//format for sending message with player state and name and message
format(msg, 128, "%s %s (%d): %s", pstate, pname, playerid, msg);

Now loop then get player vehicle id before sending message so it'll only send to those who's in the car

pawn Code:

for(new i = 0; i < MAX_PLAYERS; i++) {
        if(IsPlayerInVehicle(i, GetPlayerVehicleID(playerid)))
        {
            SendClientMessage(i, -1, msg);
        }
}

This is just a sample! don't just copy and paste.