Pulling HTML into an HTML doc from external location

[Grizzlyoomb]

Forgive me if this is too elementary for this forum.

I post auction listings that consist of simple html. I may have as many as 300 active at one time. Within the html of the particular auction ad, there is a simple left hand navigation with text links to different categories in my online store mainly used for cross promotion. I need to be able to dynamically change the navigation in the existing 300 ads when the categories change.

I was thinking that I would have a text or html file on my server containing the navigation html so I would only need to change one file and have it dynamically pulled into the auction listing when it is opened.

I assumed I could accomplish this through javascript in the auction listing. I would need to know where to get the java script. Does anyone have a better idea or need more info?

This seems like it is a simple problem, but for a hack like me, I don't know where to start to find the answer. What do you suggest I do?

Thanks for any help.

Griz

[JPnyc]

What's the serverside language involved? HTML includes or php includes would suffice.
Here's one tutorial on the html type ssi, and the php type is

ob_start();
include("/www/pathToFile.html");
$myNav = ob_get_contents();
ob_end_clean();