Wow, that's really all there is to it? Any idea why bimmernav.com would have suggested a "retrofit harness plug" and a "radio module"?

There are a few Mk4 DVD Nav computers on ebay in the $700 range and it seems that the monitors go for about $550 (far less than bimmernav.com's $2,800 to start). Other than a surround bezel and the DVD is there anything else I'll need?