I do not recommend a hardware hack for using tv while driving. Some of the hardware hacks set the driving speed to null, which interferes with other vital on-board systems.
I hacked my tv using navcoder from KiwiJochen (check google). You need to have a hardware interface (USB to X5) and license the navcoder software. Here you can also do much more than just hack the tv.
Bear in mind, that you may need to have the tv module installed; depending on your region, you need to have an analog or digital tv module (in Germany, we don't have any analg tv anymore). There are a number of good ZEMEX devices or AMPIRE devices
check
BMW Ipod Video/DVB-T Set für BMW e39,e46,e38,X5 e53,X3 bei eBay.de: Einbau-Navigationssysteme (endet 30.04.11 14:11:55 MESZ)