One trick that some people use when fitting a new wiring loom is instead of disconnecting every wire from each switch, lamp etc. is to cut off each wire leaving a couple of inches of original wiring still attached to each terminal of each electrical unit to act as a colour guide when connecting up the new wires.
(This, of course, effectively writes off the old loom, which I'm not in favour of, because 1) it can't be passed onto someone who might need it and is prepared to repair it as necessary, and 2) it wouldn't be the first time that someone bought a new loom, then found it wasn't the correct one/was badly made and decided instead to re-use their original loom....).
Another trick is, before pulling wires out of any enclosed area e.g. door pillars, that you attach a wire or similar to the end terminal, pull this through, then detach this guide wire and leave it in situ to use to fish the new wires through the enclosed section when you come to fit the new loom. I don't think there's much need for this trick on the 500 except possibly the door pillar interior light switches.
If you buy a new wiring loom, check it against the existing loom before attempting to install it - check it has the correct number of wires/terminals for each area/unit of the car before ripping the old loom to ensure that 1) you've been supplied with the correct loom for your model of car, 2) it's of acceptable quality, with same gauge of cables used.
Don't be surprised if some of the colours don't exactly match those of your correct model/year wiring diagram , the same might be true of your existing wiring loom....
A new wiring loom won't solve all electrical issues - you need to check/rectify as necessary any earth (ground) points as you proceed. It's a good idea to do a resistance check on all switches before refitting them to avoid problems.
If you fit a new loom and want to test it with the engine not yet re-installed, just insulate the main cables that connect to the starter motor and generator, hook up a battery and test away.
AL.
