Working with multiple categories
I figured it out! 

If anyone is stuck at the same place here’s my solution:

{# category is 'ford' #}
{% set entries = craft.entries.section('support').relatedTo(category).find() %}
{% set partsCategories = craft.categories.group('partsGroup').relatedTo(entries) %}

{# Partslist #}
{% for partsCategory in partsCategories %}

    {{partsCategory.title}}
    {% for part in craft.entries.relatedTo([
      'and',
      { targetElement: partsCategory },
      { targetElement: category }
    ]) %}

      {{part.title}}

    {% endfor %}
{% endfor %}