Fabian Keil - 2015-05-03

The search function uses a https:// URL so Privoxy will not see the
path and can't use its content for redirects.

Allowing Privoxy to filter encrypted content is on the TODO list:
http://ijbswa.cvs.sourceforge.net/viewvc/ijbswa/current/TODO?view=markup#l57

In the meantime you could replace the search URL itself using a content filter.