I think I've worked out what's wrong, and the various boxes are in the wrong place. (David's post and the photograph of the J.D.3 between the Binoculars and Signal cartridges also helped.)
Assuming that's "upstairs", i.e. in the turret, it's definitely in the wrong place, because that's for the Driver and Bow Gunner down in the hull. The cable from the JD3 connects to the Hull Junction Box for access to the Rotary Base Junction. Connections are Speech (Intercom output), Signal (buzzer call to commander's droplead), Mic+ & Mic- (for intercom), 12V (for buzzer) and earth (for buzzer and intercom ground).
Because it's currently located between the Binoculars and Signal Cartridges, I'd guess that's the Commander's position, and should have Control Unit No.1 (which has a single 12-way connector, two dropleads (commander and gunner) and a switch that can select A IC B. It will also have a cable entry for a 4-way lead that connects the turret junction box to the terminal board in the control unit: Signal, Speech, Mic+ & Mic- (the cable entry can come out of the top of the box, next to the 12-way connector, or from the right hand side - unused holes will be blocked by rubber grommets). A 12-way cable runs from the connector on the top of the box, round the back wall of the turret to the Radio Operator's Box (Control Unit No.2) which has a single droplead, two switches (A IC B and N R), a red warning light ("A Set Unattended") and two 12-way connectors.One of the 12-way connectors goes to Control Unit No.1, the other goes to the Wireless Set.
Power supply: the cable used depends on the vehicle voltage (the WS19 itself requires 12 volts) and the power unit fitted. It's a late-WW2 vehicle, so has a 24 Volt battery (probably) and Wireless Set No.19 Mk.3 (probably Canadian, with Supply Unit No.2). That can have 3 options: 12V 2-wire feed, 24-volt 3-wire feed or 24 volt 2-wire feed, and they require different connections to the turret terminal box. Getting it wrong can cause serious damage to the set - like "all the valves and lamps will have to be replaced" damage.
What set and supply unit do you have?
Best regards,
Chris.