As I notice an older video of mine gets quite some traffic, I have reasons to believe that some people here could benefit from it too (originally made in Dutch, but English undertitles are available). Applicable to the Zümo 595 and 590 (same support). And I even think it also applies to the 395 and the 390, but this has to be confirmed.
Parts list included in case people want to order online the parts used in this video.
Take care, of course : other signals aren't available any longer (which is the logical consequence when you want a less cumbersome cable loom). But you can still have the GPS's spoken commands over the (bluetooth) intercom if you have one available...