Somone...

Yes, me

... in a former thread traced a way to define links as opening in a blank...

Not in 'a blank' but in a new browser window ...

... can anyone explain in which files these changes should be done? I tried it with the functions.inc.php but it gave me lots of errors.

I don't want to do your homework, but you have to look for potential links not only in *.inc.php but also in *.tpl.inc