Chuck, I'v found something strange :
when I click on this link
http://www.plouf.be/scuba-planet/sho...oduct=5&cat=14 in showproduct.php the code found " elseif( !empty($date) )" and the select was running that :
SELECT id,bigimage,cat,userid,approved FROM {$Globals['rp_db_prefix']}products WHERE date >= $lowdate AND date < $highdate AND approved=1 $whichcat ORDER BY date DESC
This select only retrieve product by date and not by manufacturer.