Why you put the below lines in 'relations()' function?
Yii::import('application.modules.user.models.*');
Yii::app()->getModule('user');
You need put 'brand' how a public or private attribute on 'Product' model. This attribute is not persistent, only use for the get the filter form value.
Then use like this:
$criteria->compare('user.brand', $this->brand, false);
And is possible that you need put 'brand' attribute on the 'rules()', on the array() with 'on'=>'search'.