Replacing the Wiring Harness.

Special tools: Semi-decent crimping tool preferably with ratchet operation, insulation stripping tool and soldering iron.

When replacing the wiring harness take the opportunity to increase the length of the wires slightly, so you have more access to the instruments, also you can add extra circuits, new fuse box etc.  This method can also be useful if you wish to just re-wire the engine bay for example and want to cut though at some point in the harness.

Where to start..... First get a copy of the Vehicle Wiring Products Catalogue or similar.  Take one old cracking harness, preferably without any major bodges.  Make a note of what goes where, helps if you have a second harness.  Cut away the majority of the old wrapping tape, but leave a little around the major branches or zip tie around the branches.  Measure up each color and diameter of wire....make yourself an Excel table up (over compensate slightly, if you want circuits a little longer take this into account).  Count the number and type of connectors.  Decide if you want a new fuse box (VWP do a 16 blade fuse type etc demon tweeks do some great fuse boxes) and if so where it will be positioned.  Decide if you want new connector blocks etc.

Once you have your table and know all the connectors you are going to need etc, get yourself a sheet of chipboard/ply.  Lay the old harness on the board and at each branch, (and along straights where necessary), screw a pair of screws either side of the harness.  Helps if you protect the screw threads with a drinking straw cut down or heat shrink.  Mark at each end of the branches, what it is for and what color and diameter of wires you expect to be there.  Then remove the old harness, tie elastic bands tight between the pairs of screws and take your new wires and (with the old harness nearby for guidance) lay/thread the wires through the pairs of screws until they reach from one end to another.  Add approx 10cm for readjustment and in certain cases to increase the length of an individual branch.  As you thread the wires through the pairs of screws the elastic bands stop the wire from springing out.  Then once ready, either tape it up or fit it briefly to make sure.

The harness is now ready to add connectors and blocks, just crimp, solder and heat shrink the majority of the connections.

Whilst doing the harness it is a good opportunity to add in some extra lengths of wire. While these may not be currently connected to anything to start with if you find you want to add an extra horn/light/etc there are wires already in the loom rather than you having to trail another couple of wires here and there and cut into the new loom you have just assembled. 

Hope this helps anyone who might be thinking of rewiring.  Far easier than swearing and cursing trying to find the one bad wire you are after, a bit more expensive, and some special terminals used.  But peace of mind in the long run.