Do they get written off by the insurer, or do they get re-used after a bench test?
Does anyone here have any first hand experience?
We had a PA28 last year where the genius pilot did not think checklists where worthwhile and forgot to switch tanks. On short final, the tank went dry and the airplane crashed (no injuries but airplane beyond repair). Not long thereafter, I saw a crew of people with a trailer arrive and disassemble the plane in less than a day.
Turns out they were a specialized company that bought the remainders including everything inside from the owner with the insurance company's approval. They take out everything that is of use and market it. The GNS430 and the King HSI were the major factors the guy mentioned.
Does anyone here have any first hand experience?
Back in the days when I co-owned two aeroplanes, we bought quite a few spare parts (including avionics) from wrecks. Our maintenance shop used to buy the remains from the insurers, and sell them piece by piece. I specifically remember that we got new reading lights and aircondition-vents for our C421 (already impossible to find ten years ago) from one that some idiot tried to land at Courchevel. Our maintenance guy drove there with a truck and two mechanics and they didn't even need to cut it up as it lay there in handy pieces already (luckily no fatalities).
Huge market in it. Avionics and every other valuable item.
Avionics and other components are definitely reused. This is the key reason that the installer must have the previous certification. If the unit has been subject to an unusual event, this should be recorded in that certification. This will allow the installer to assure that the unit meets the airworthiness requirements for installation in another aircraft.
A radio which was simply on board during a crash is not automatically unairworthy, it was just near the wrong place at the wrong time. It can be bench checked, and recycled. My Teal is about to have a very nice King Comm installed, which I purchased and removed from a 172, whose pilot misjudged his fuel aboard. The resulting crash wrote off the plane, and caused minor injuries. I am told that fortunately, his wife, who was slightly more injured, did not remember the crash at all, as she has Alzheimers. When I removed the radio stack, which was beautifully installed, I found his car keys wedged up in there - it must have been quite a tumble!
What is the exact procedure, under N-reg or EU-reg?
One EASA145 company owner told me he would re-use such parts only if the plane they came out of had a valid CofA at the time of the crash.
Obviously the CofA won't be valid after the crash, however, when you come to remove the part 
UK avionics installer opinion seems to be that an EASA-1 form is needed to install anything whatever, which means the item has to be processed via an EASA145 company with the right scope for recertifying it. The equivalent process under FAA would be a FAR145 Repair Station (generating an 8130-3) but I am sure an 8130-3 is not automatically required.
More practically, I would be concerned about cracked PCBs, etc. You can get all sorts of intermittent problems.
In the past, I used to run the Honeywell extended warranty. I replaced a few bits under it but stopped paying for it quite soon because it was so obviously poor value. I wrote up some notes on it. In short, it was obvious that stuff supplied under it was all old (10-15 years on average) but was being invoiced at extortionate brand-new-plus-a-bit values, which made the warranty look like worth having 
But the key point with that pool of "warranty swap" avionics was that a lot of them were duff. I reckon 1/3 of them had either intermittent problems, or subtle permanent problems of a sort most pilots would not notice, or would not notice during the warranty period. I have no evidence to suggest any of this warranty pool came out of wrecked planes, but it might have done. Certainly some of it came from owners who had intermitted problems, kicked up a massive fuss, and got the item swapped out. PROB90 the intermittent item would bench test OK.
