I am not sure that this is the best approach to my problem, but I believe this should work.
I have a Product Brand page that links to a Product Line page.
The Product Line channel contains a Relationship Custom Field called product_brand which is related to the Product Brand channel.
Here is the code block in my Product Line template.
The following code has been simplified for readability (remove HTML classes, IDs, etc.):
{exp:channel:entries channel="product_line" search:product_brand="Brand Name"}
<!-- I know this will not be Brand Name. It should contain the string from the relationship drowdown
right? Placing not IS_EMPTY returns all entries under Product Line regardless of their relationship
or lack of a relationship -->
<div>
<a href="#">_ {product_line_image}_ </a>
<h3><a href="#">{title}</a></h3>
<p> <br />
{product_line_description}<br />
<br />
<a href="#">Learn More</a><br />
<br />
</div><br />
<br />
{/exp:channel:entries}Can I not reliably retrieve the value of a relationship custom field?
Please let me know if this is confusing.