My Bongo is - 2000 V6 converted (ex)flat-top with original spoiler (else YMMV) ...
With internal seats and headrests, I found that, at best, I had a letter-box view behind me. With kids in place, view went to nought! After a couple of (very minor) dings reversing, and a couple of near misses with the lack of rear-view, I thought I'd try 'new-tech'.
Got a 'bracket' twin-camera with clip-on monitor and 5m cables from reversingcameras.co.uk (not a plug, just a fact).
Spoiler has T30 star-bolts to hold it on ... I've never met these before, and it took a while to tool my kit. They undid nicely. No torque and the spoiler just lifted off (slight 'tick' as it had plastic thingys to orientate it - 'straight' pull in direction of bolts all that was needed). Connector block for high-level brake-light wiring is obvious and easy (didn't just rip it off).
Rear mirror (whose functionality was being replaced anyway) took serious worries to undo ... WD40, patience and a VERY well fitting driver head did the trick. Only stripped one head
. 10mmxM6 bolts fit fine!I'd measured and made up a plate of 2-3mm aluminium to mount the camera body onto, and the angle/size seemed fine. Bolted that on using the original rubber from the mirror.
Cables for the cameras came in pairs (one each!). All the rest is for 2 cables as there are 2 cameras.
Decided early that cables would come down off-side to driver-side wiring at the front. Started by taking off the plastic trim stuff on rear pillar ... pain, and I still have some clips (anyone got any advice about taking the stuff off - I'm not a mechanic).
There are a couple of holes in the internal skin which allow access to the single hole coming out of the back - one is empty (?) and the other takes the tailgate wiring.
Tailgate wiring runs from the outer skin to the tailgate via a tube with grommets at each end. I couldn't get the camera cables through this, so I snipped a hole in each grommet and ran the cables outside the tube (will use some silicon sealant to restore integrity as this is a water-entry point).
Coming out of the tailgate there's a plastic thingy, which the wires for the brake-light go through. I snipped this to allow the camera cables to go through. Cables fitted into the space nicely, I just couldn't thread them through.
Spoiler is hollow and (relatively) easy to take cables through ... my neighbours may have a different impression!
The brake-light on the spoiler is made of 2 parts - rearmost is the actual LED-strip, and t'other bit is just a soft plastic fitting (again, YMMV). There is a hole in the spoiler which allows the wiring through, that is big enough to ...! I made a hole in t'other bit to feed the cables through.
Cables now run up rear pillar, through a hole, out at the edges of the tailgate wiring grommet and out of the outer skin of the tailgate beside the brake-light wiring. From there they go into the spoiler and then out of a (created) gap in the brake-light fitting.
No holes drilled. A small amount of (black) silicon sealant to repair holes in rubber seals.
5m cable not quite enough, and it's difficult to get spoiler off/service cameras etc, so I'm going to put a 1m extension cable inside the spoiler to allow disconnection etc.
Now to put it back together again - bl**dy trim!
All works. Enjoying
mine at the moment are not night vision, what would I need to adapt it to plug into a TV or VCR 
Spherical Visions