The code you posted will look for rssfeeds.php in the root directory of your site. If the file is actually in /buynewcar, change the path:
You also seem to be using a .html file extension. SSI, by default, uses the extension .shtml. If you want SSI to work on .html pages, you need to add the following to /.htaccess:
AddHandler server-parsed .html
(this assumes you aren't already using SSI elsewhere and haven't already done this)
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