Changing siteurl to a different directory

i have my wordpress installation in a subdirectory (site.com/wordpress), and then fixed the index.php and moved it to the main directory (site.com) and all was working fine..

i now need to load the site from a subdirectory of the mainsite (site.com/subdirectory) and I’m trying to keep the wordpress installation where it is.

so i changed the siteurl in settings to site.com/subdirectory

however, when i changed the index.php to go to site.com/wordpress/wp-blog-header.php i get some errors when opening site.com/subdirectory

i’m thinking it’s because i put the whole address instead of a relative path.. can someone clue me on on how to do this? i gather that ‘./’ means subdirectory.. so how do i direct it to a completely different directory?