Where does a type specific field go? Let's say I have table of cars and table of car manufacturers. Each car references one manufacturer.

Furthermore, each car that has manufacturer x requires an extra field. Say, cars from that manufacturer have an extra feature that needs configuring that no other manufacturer provides.

Where does this field go?

I think there should be another table with a one-one mapping between it and the cars table which holds the extra field for cars of that manufacturer.

So given this, how do I make sure that only cars of the correct type have this extra data in the other table? Or is this really bad data design?
The features data should really exist in a separate table.  Normalize your data for maintainability and scalability.  For instance, you could implement something like this:

create table Manf
(
    pkManf int identity(1, 1) not null primary key clustered,
    Name varchar(256) not null
)

create table Car
(
    pkCar int identity(1, 1) not null primary key clustered,
    fkManf int not null foreign key references Manf(pkManf),
    Name varchar(256) not null
)

create table Features
(
    pkFeatures int identity(1, 1) not null primary key clustered,
    fkManf int not null foreign key references Manf(pkManf),
    Name varchar(256) not null
)


Just because you currently only have one manufacturer with a feature that needs to be noted in data, that doesn't mean that's always going to be the case.  It is a very easy SELECT to retrieve all of the cars with a particular feature.

EDIT: To get all Cars with a particular feature, you'd do something like this:

select c.*
from Car c
inner join Manf m
on c.fkManf = m.pkManf
inner join Features f
on f.fkManf = m.pkManf
where f.Name = 'some feature'