Hi again.
I think I have identified the relay for you, I think. It's under the glovebox, near the fusebox and is coloured red and I THINK it has two unused electrical pin holes in the top of it. It is next to a big yellow relay. Hope that bit helps.
My new auxiliary socket had a fresh permanent live supply. Took it from heater blower permanent live, under driver's glove box, fuse 5. Big thick red wire. Can't miss it.
Installed a new socket fitted on the little shelf on drivers side cos as I said, my two factory fitted ones were still working on the ignition.
There is a useful allen key bolt near the fuse box which supplied an earth and I fitted an illuminating switch alongside the new socket to turn it on and off. Used new wiring for new switch. Quite easy really.
I have copied my Yahoo post here for anyone interested.
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Just had a successful morning wiring in a permanent live auxiliary
socket on my '57 nosejob Multi. I wanted a permanent live socket cos
I am a chauffeur and I spend a lot of time just sitting in the car
and don't always want the engine running or ignition on while I'm
just watching me little telly or dvd etc.
Hardest thing was to find a decent and accessible permanent live.
After and afternoons probing with a multimeter in the fusebox area
under the driver's side oddments tray, I only succeeded in finding
the wires from the fuse box to the courtesy lights, the radio memory
live (behind the radio) and a large red wire controlled by fuse no. 5
which according to my handbook covers the interior fan circuit. Why
that needs a live feed, goodness only knows, but hey, thanks Fiat!
I was initially wanting to find the horn or even the hazard warning
light circuits because these are safe circuits if I cocked it all up,
but no amount of probing, so to speak would bring them to my
attention. I even had the steering wheel cowl off to see if I could
see what colour the wire to the horn was but couldn't find a live
one. Didn't want to poke around there too much in case I set the
airbag off!
I felt that the courtesy wires were to small to take a feed off and
the radio live was a bit inaccessible.
In the end I settled for the large red wire on the fuse 5 circuit. If
as the handbook says it covers the interior fan only, it's a perfect
source! It's on a 30 amp fuse.
Because I fancied doing a fairly neat job, I decided not to cut into
it, but to bare the wire, tin it for soldering and solder my fuse
holder for the auxiliary socket to it. I wanted my fuse for this new
feed to be in with the other fuses for ease of replacement and
continuity.
So I soldered onto this live feed (with its fuse removed!). I then
soldered the other side of the fuse holder onto a wire for supply to
an illuminated switch so that I could make the socket switchable and
have a visible indication that the socket was live. I then made an
earth onto the frame under a handy allen screw in the fusebox area
using a ring connector.
Next I made holes to mount the auxiliary socket and illuminated
switch. I decided to mount them just above the handy little shelf
under the dash on the driver's side. Bit unnerving this cause I was
drilling a 30mm and 10mm hole in a year old car I had only just
bought! This was a useful place to mount the socket because there is
a lot of space behind here for the socket to go through, it is
adjacent to the fuse box and if I am charging my phone from the
socket or something, it can lie on the shelf and wires won't trail
across anything.
After this it was really just a case of connecting the supply and
earth to the switch and running a supply and earth from the switch to
the socket. I tested the circuit before mounting the switch and the
socket and all worked fine. I then just mounted the socket and
switch, tidied up the wiring and tested the socket under load with a
dvd player, phone charger and mini tv. I left the dvd playing for
half an hour while I cleared up and there were no blowing of fuses,
overheating of wires or visible evidence of any problems.
The fuse 5 circuit I tapped in to is fused at 30amp. I fused my
circuit at 7.5amp so that if I plug anything dubious into the socket,
my fuse should blow and not affect the fuse 5 circuit. I could
probably comfortably have fused at 10amp and been fine (the radio is,
after all). Anything requiring more amperage than this I would not
want to run from a permanent live supply anyway, but use the onboard
supply sockets and have the engine running so as not to flatten the
battery. I believe these are fused at 15amp and have a 180watt
maximum loading. But I also believe they have a relay, and my circuit
doesn't.
Apart from the previous afternoon spent poking around the fuse box,
the actual fitting and wiring took about two and a half hours. I took
my time and tried to keep everything looking neat and tidy. The
socket was from Maplins made by Hama and has a nice protective cover
to the front to prevent you poking your fingers in etc. And the
illuminated switch was from Halfords.
Socket £4.99, illuminated switch £2.75, Fuseholder £2.00, connectors
and cabling probably another £5.00. All in all for about £15.00 and a
couple of hour or so's work a useful addition to the car.
I am not suggesting that what I have done here is the absolute
safest, neatest or best way of doing it. But I am comfortable that
what I have done is definitely safe. I have only tapped into the
interior fan circuit, not exactly life threatening! I used this same
circuit as I am using now, for over three years in my old uglybug as
a live auxiliary feed and I fused it at 15amp cos I wasn't aware of
what I was doing. In all that time it caused no problem whatsoever so
I am confident that it safe to use for this purpose. I have fused it
down to make it so, anyway.
Hope this helps.
Panda.
Ha ha! I posted it on the yahoo forum, here: 
