It is possible, but its a major pain in the butt... A lot of shops won't do it, and the ones that will charge a lot of money. The ones that do it correctly, put a sensor that will only let it work when the parking break is set, and the transmission is in neutral.
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!