First, to clarify, I am answering this based upon DBIx::Class and the belongs_to method contained in the DBIx::Class::Relationship submodule.

Regarding null foreign keys, the documentation says:

If the relationship is optional -- i.e. the column containing the foreign key can be NULL -- then the belongs_to relationship does the right thing. Thus, in the example above $obj->author would return undef. However in this case you would probably want to set the join_type attribute so that a LEFT JOIN is done, which makes complex resultsets involving join or prefetch operations work correctly.

If I am reading your question correctly, it sounds like this is your difficulty. The solution can be implemented (based on the module documentation):

My::DBIC::Schema::Car->belongs_to(radio => 'My::DBIC::Schema::Radio',
                                  'radio', {join_type => 'left'});
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If this doesn't help, post your calling code and we can see if the issue lies there.