[Emmet_]

pawn Code:

public CheckOwner(playerid)
{
    if(IsPlayerConnected(playerid))
    {
        for(new i = 1; i < sizeof(CarInfo); i++)
        {
            new playername[MAX_PLAYER_NAME];
            GetPlayerName(playerid,playername,sizeof(playername));
            if(strcmp(playername,CarInfo[i][vOwner],true)==0)
            {
                PlayerInfo[playerid][pVehicle] = 1;
                return i;
            }
            else 
            {
                PlayerInfo[playerid][pVehicle] = 0;
                return i;
            }
        }
    }
    return 1;
}

You should use 'else' and not this:

pawn Code:

else if(strcmp(playername,CarInfo[i][vOwner],false)==0)
            {
                PlayerInfo[playerid][pVehicle] = 0;
                return i;
            }

That checks if the player's name IS the owner of the vehicle again!