It would be basically impossible to put together any functioning electrical system without wiring diagrams and tools. It's unlikely that Average Joe... or for that matter... the average aircraft engineer.... could wire up the remains of an electrical system without these. The electrical connectors on the back of some of the radios and computers on any modern jetliner have dozens of electrical pins/sockets.... How do you know which is which, except perhaps by smashing them open with the fire axe... Then you run the risk of damaging the circuits inside (Unfortunately, Swiss Army Knives with their trusty screwdrivers are no longer allowed to be carried by passengers.... but you may find some in the cargo hold in passenger's bags). Whatever you did, it would still come down to a long trial and error process... and you may risk damaging the circuits by putting the wrong voltages on the wrong components.
You would never get the APU running (as it is controlled by a computer with masses of interconnecting wiring). You would probably flatten the aircraft battery attempting to start it, when you could put the battery to better, longer term, use.
Your best bet, as previously mentioned, would be to get the emergency locator beacons working (some are mounted close slide rafts, others in cupboards near doors). Failing that, I suppose you could blow a slide raft and paddle/sail to the nearest populated land mass? (you could make a sail from thermal insulation blankets).
There should be enough easily removable interior lightweight panels and thermal insulation material to make a decent waterproof shelter if the fuselage was in too many pieces to offer shelter. The seat cushions are removable, so you could make beds out of those if you couldn't figure out how to operate the manual override controls on the electrically-powered first class recliner chairs :-)
Cabin emergency lights have rechargeable battery packs. Hopefully not all of them will have been triggered to operate when the aircraft lost power.
Knowledge is everything. There are probably lots of things which an aircraft engineer could use... but would the average passenger recognise these things and know what to do with them?
As previously mentioned, steer clear of control surfaces. The depleted uranium balance weights should have been replaced with tungsten on aircraft still in use ... but your scrap aircraft may still have it inside. The same applies to emergency exit signs, toilet smoke detectors and other devices (they may contain small amounts of radiactive material).
BTW, who survives the crash (any flight/cabin crew members?).... or would that be giving the plot away?
Rgds.
NSEU