Hi,
A friend of mine runs a the Beier Electronic USM-RC-2 Sound unit in his 4200 xl,
http://www.beier-electronic.de/model...2/usm-rc-2.php
Just a quick note if you go to order one, buy it with the cd but with on the user manual as user manual that come with it is not in English and then just download the user manual you want off there website.
I also have this sound unit in my Truck and I love it, I ended up ordering my speaker and speaker box from the Australian Servonaut dealer Kevin from Csmi,
http://www.csmi.com.au/
Regards Cameron.