On the Anchor Bay DVD of Maniac, select "Extras" from the main menu. Press "right" on your remote and you'll highlight the "X" on the woman's head. press your Enter button and you'll be treated to an outtake from the audio commentary of the film The Guardian. In it, you'll hear William Friedkin extol the virtues of Willam Lustig and his little gory masterpiece.
Note that this feature is not available on the Image Entertainment DVD. It also doesn't matter if you have the regular edition or the "Tin Box" edition, they're both the same.
All easter eggs were generously submitted by SirDonk