Custom Query Content Filtering

I assume that your posts are having make and model as a Custom Fields and you want to retrieve all those posts which are matching both the passed value for make and model Custom Fields. If that is the case then your code seems to be correct. It might be possible that there is no matching post is there so you are not getting any result. But if you wants to retrieve all the posts which are either matching make or model Custom Fields value then you might need to add relation parameter in meta_query as the default value for the relation parameter is AND. Below is how you can add relation parameter with the OR in code.

'meta_query' => array(
    'relation' => 'OR',
    array(
        'key' => 'make',
        'value' => $make,
        'compare' => '='
    ),
    array(
        'key' => 'model',
        'value' => $model,
        'compare' => '='
    )
)

Check the following link get more details on using meta_query in WP_Query. https://codex.wordpress.org/Class_Reference/WP_Query