Radio/CD unit not working

[brianjeff50]

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?

[g8dhe]

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