Here are some suggestions:

The title/heading of the first menu layer, 'Search Nodes', is confusing. It might be better just to echo the link the user just clicked to slide out the menu i.e. "Add Nodes" 

It feels like you might be over-using checkboxes. The first example of this is next to 'Select previously bookmarked nodes'. A checkbox shouldn't be necessary if there's only one option, and even so, you complicate the interaction here. A checkbox should require one click to select, and another to confirm the choice, whereas a more appropriate element, like a button or a simple link, would only require one. 

Regarding your key question, again, I think your choice of repeating checkboxes and buttons will result in complicated interactions and cluttered UI. 

Here's an alternative solution for your consideration:



(1) An accordion-style menu, with disclosure triangles to reveal deeper options.

(2) Hovering over elements that can interacted with reveals a text link and action. In this case a branch node has already been added (hence the green color and checkmark) so the option is to Remove.

(3) This node has already been added.

(4) Hovering over this leaf node reveals the option to Add.

Of course there are are lots of stylistic variations to this, but it doesn't require multiple slide out menu trays, both leaf and branch nodes can be added/removed, and it doesn't require the repetition of check boxes and buttons. 

How to 'select' menu items that also link to deeper items?
The vehicle is a 2003 Chevrolet Trailblazer. The low beams aren't working. High beams are. The fuse is perfectly fine (tested and swapped out). The bulbs are fine too.

A mechanic told me it could be the "body modulator". I've never heard of this before. Could this be the problem? If so, how hard is it to change out for a shade tree mechanic?

If not, what else could this be? 
