The light at the front fastened to the windscreen frame,
The one at the back was different. Im not sure what the back of the cab's like on the defender. On the series its sort of recessed so I cut a bit of hardboard with the hole for the light The harboard is a tight fit by where the galvanised bit in the photos is and theres about 2" of space behind it for the wiring and things.
For some reason I thought you had a truck cab (me misreading), its why I didnt mention it earlier