public OnPlayerDeath(playerid, killerid, reason)
{
    if(IsPlayerConnected(killerid)) // Checks if your killer is connected.
    {
        if(GetPlayerState(killerid) == PLAYER_STATE_DRIVER) // Checks if the persons state is in the driver seat.
        {
        // Code here, btw the Playerid for the person who car killed is "killerid"
        }
    }
}