Step back a second ....

Zen Cart's infrastructure auto-selects which template to use based on the URL parameters.
It starts from index.php
In the case of the URL you're proposing: index.php?main_page=index&cPath=66
...
main_page=index tells it to use tpl_index_default
but, since cPath is specified, it flips over to using either tpl_index_product_list.php or tpl_index_categories.php and a few others such as tpl_modules_category_row.php and so on.

But, before that even happens, all pages start from tpl_main_page.php, which builds header, left-sideboxes, then main content from the page template (just mentioned above), then right-sideboxes, then footer.

By adding the code suggested earlier, the dmode=inline switch will cause tpl_main_page to "only" display the content for the category number 66.

Thus, you don't need to reference a specific template/template-file. You just use the URL.

As for what to put in the iframe SRC parameter, I suspect that the URL you mentioned in your last post will be fine. It's likely wise to use the complete URL, including http: //domain_name etc...