including custom fields in the_content for posts

I’ve been struggling to include custom field content into the_content.

I came across this helpful post using add_filter to hook into the_content.

http://wordpress.org/support/topic/290591?replies=3

I created a content modifier function and added it to the functions.php file;

function custom_content($content) {
	global $post; global $wp_query;
	if (is_syndicated()):
		$vidstr=get_post_meta($post->ID,'Video', true);
			if (!is_null($vidstr)):
				$findstring="&feature=youtube_gdata";
				$findit=(string)$findstring;
					if (strpos($vidstr, $findit) !== false) {
						$vidstr=str_replace("&feature=youtube_gdata","",$vidstr);
						$vidstr=str_replace("http://","httpv://",$vidstr);
						$extracontent = "<b/>".$vidstr;
						}
				$findstring="(/feeds/base/videos/";
				$findit=(string)$findstring;
				if (strpos($vidstr, $findit) !== false) {
					$vidstr=str_replace("gdata.youtbue.com/feeds/base/videos/","www.youtube.com/watch?v=",$vidstr);
					$vidstr=str_replace("http://","httpv://",$vidstr);
					$extracontent = "<b/>".$vidstr;
					}
			endif;
	endif;
	$original = $content;
	$content .= $extracontent;
	$content .= $original;
return $content;
}
add_filter('the_content', 'custom_content');

The code above works if I add it to the single.php file directly, however the content will be outside “the_content” where I need to apply some plugins to the output.

and in the single.php I just have

the_content('');

However, no matter what I do I cannot modify the_content to include the content modification above or any other modifications, no matter how simple.

Could somebody point me in the right direction please ?

Thanks you.