Install wordpress under www.mydomain.com/wordpress

hello all!

I have a store website under http://www.mydomain.com, and I want to install wordpress under http://www.mydomain.com/wordpress in order to add a link in the store to the blog.

The problem is:
http://www.mydomain.com/wordpress/ ->OK
http://www.mydomain.com/wordpress -> is redirected to http://www.mydomain.com/foldername/wordpress and 404 not found.

To solve that I added a rewrite rule which adds a trailling slash at the end in the htacces under the root folder and everything worked well:
Options +FollowSymlinks
RewriteEngine On
RewriteRule ^/*(.+/)?([^.]*[^/])$ http://%{HTTP_HOST}/$1$2/ [L,R=301]

But when I activate permanent links the problem occurs again because I need to have a .htaccess file under the folder /wordpress, and then the previous rule in the main .htaccess file doesn’t apply any more.

Any idea to merge these two htaccess files in one to get this working??

thanks a lot!!