There may be alterative way to get a Phatbox working in the Escalade. My own setup is a Kenwood Music Keg interfaced directly to the stock GM stereo in my 2004 Chevrolet Impala using a PIE interface. The setup works well, except that the display on the GM head unit is very simple (like D1T15) for disk one, track 15, for example. See the Digital Protocol Converters section at
www.pie.net. I believe that vehicle would need the a different interface. I am using the GM12/PC-KNW, and that model year requires a GM9/PC-KNW.
You could also buy and use the Kenwood FM Modulator, and then use a PIE or other GM to Aux input converter and directly connect your PhatBox to the factory system. You would have to find a place to mount the modulator display, but it would give a much better display of your song characteristics.
Finally, you could change your stock head unit to a Kenwood, but you would need a special box to preserve your warning chimes, which come from the factory sound system in these GM cars. If you remove the factory radio you also loose the ability to customize settings for the automatic door locks, etc. You can still reset the oil life monitor and tire pressure monitor using methods other than the buttons on the stock radio. I belive there are harnesses for these older GM cars that allow you to relocate the radio to preserve the chimes and ability to change the car's configuration.
I have never seen the genuine GM Phatbox. I suspect it will be expensive.