display subnav problems

Hi all,

I’m trying to make my Cars subnav stay open when on child pages.
ie:

Menu is:
– Home
– Cars (on click opens ‘Ford’ and ‘Dodge’)
–Ford
–Dodge
– Contact

Ford and Dodge Pages display thumbnails of relevant cars linking to individual pages about each car. However when clicking on a thumbnail and bringing up the individual page, the subnav menu of Cars disappears and I’m left with.

– Home
– Cars
– Contact

I want the Cars Nav to stay open on these individual page.. I’ve have a good look around – only found this which is very simular but un-resolved. https://www.ads-software.com/support/topic/show-subnav-of-sister-pages, is it something to do with the ‘depth’ => ‘3’ within the wp_nav_menu?

Many thanks in advance.