Bongo Fury

An aftermarket Sony with a clip-out front is fitted to our newly acquired Bongo but doesn't work. Seems to have power at the main feed in but otherwise dead.
While delving in the radio hole I found an unconnected black&purple cable with connector that's part of the original loom. Does this do anything?

Probably the Earth connection! for the radio and frame as otherwise the frame is insulated by the plastic dashboard.