Okay I see what you want...
These CONSTANTS are called language defines.
When I used ADMIN>>>TOOLS>>>DEVELOPERS TOOLKIT to do a search for this CONSTANT, I find that it occurs as follows:
/includes/languages/english/advanced_search.php
Line #31 : define('TEXT_ALL_MANUFACTURERS', 'All Manufacturers');
/includes/languages/english/advanced_search_result.php
Line #42 : define('TEXT_ALL_MANUFACTURERS', 'All Manufacturers');
/includes/languages/english/index.php
Line #47 : define('TEXT_ALL_MANUFACTURERS', 'All Manufacturers');
... So you'll need to edit the defines in the three files shown above.
HOWEVER... you need to FTP these files to your local machine, edit them on your local machine, then FTP the edited version s to the OVER-RIDE folders of your template.
EG:
For: /includes/languages/english/index.php
Edited file goes to: /includes/languages/english/YOUR_TEMPLATE/index.php