Most of the new decks are smarter than they once were. Many of them now require the ground to be activated after the switched source is turned on. This means if you just ground the wire, the headunit will not activate the "parked" mode and will not play videos. There are a bunch of tutorials on youtube and google on how to use a relay that will allow you to this mod correctly.
Current: 2013 Focus ST II.
Past: 06 Explorer
Past: 03 Focus ZX3 Modded: Rims, SVT Bolt-Ons, Tons of audio stuff with Kenwood Headunit, MB Quart Components, and Subthump Stealth Hatch Enclosure and Amp Rack Loaded with Kicker Goodies!