IIRC it was fitted in the cabin roof just to the right of the Nav Radar, next to his dangly grab handle.
I remember being called for a crew in snag where Nav Rad says VASU problem, I said what problem, and he says 'I haven't got one!', I looked and he did have one, but the hangar team who had been working in the cabin had swapped the locations of the VASU and Grab handle. The Nav Rad was used to putting his arm up and hitting the correct spot in the gloom everytime, it never occured to him that it could be elsewhere.