Right, it all works!
You were right about this, I was going way overboard with 6mm wire, which wouldn't have fitted in all the connectors. i guess the original fan motor ran a much higher current. When I looked the actual ratings I found 1mm wire would be sufficient for fan speed 1. Anyway, this is what the finished wiring looked like:
Which is to this wiring diagram:
Essentially the same as the one at the top of the page but I have updated the wire thicknesses. Also I couldn't find Red/Green wire in 4mm so just used Red.
I used this wire:
Thin Wall Cable (autoelectricsupplies.co.uk)
And these connectors to splice the wires:
108961-000 Raychem - Te Connectivity, Butt Splice, Yellow, DuraSeal Series | Farnell
Which are Duraseal Yellow crimp butt connectors with heatshrink. These are nice because the heatshrink material gives a good seal around the wires at the connection. You need yellow (10-12AWG) for all connections I found. The Blue connectors (14-16 AWG) is just too small, even for the 1 and 2mm wire join, which approximately makes 13 AWG. You have to double over the end of the 2.0mm wire where it is on its own, but this is no big issue.
The resistors I used are:
HSC100R47J | TE Connectivity HSC100 Series Aluminium Housed Solder Lug Wire Wound Panel Mount Resistor, 470mΩ ±5% 100W | RS Components (rs-online.com)
For the fan speed 1 circuit. This is a 0.47 Ohm resistor as I couldn't find a 0.4 Ohm. Makes little difference though. Don't know if this is the best brand, but it seemed to have a better temperature stability.
HSA50R10J | TE Connectivity HSA50 Series Aluminium Housed Solder Lug Wire Wound Panel Mount Resistor, 100mΩ ±5% 50W | RS Components (rs-online.com)
For fan speed 2.
At the end the resistances were about 0.7 Ohm for fan speed 1, 0.2 Ohm for fan speed 2 and 0.1 Ohm for fan speed 3. Little bit higher accounting for the extra resistances for the other circuit components, and possibly rounding due to my multimeter only going to 1 decimal place accuracy, but within the right original range.
You can also get the female connectors for the Comex slimline fans here:
2 Pin Plug Connector for Comex Fans from Merlin Motorsport
For straight plug and play. These (and the spade connectors on the Passat plug) are open barrel types so you will need the correct crimper tool. To get the existing wires out of the Passat spade connectors is a pain but doable. For the 2 larger connectors (Speed 3 and Ground) I found it was possible to hammer a small screwdriver (jewellery/watch type) down the back of the wire to open it up. Make sure the spade is held securely in a vice to stop it bending, and use pliers to hold the neck of the connector as well. For the smaller two it should be possible to pull the wires out with a pair of pliers, starting with one or two threads in the middle of the bunch. With the wires out you can just about open the metal enough to get the new wire in. Because you are reusing these I would solder as well as re-crimping them. I re-crimped and the seemed to be holding OK, but soldered as well as a back up.
I then knocked up a bracket to mount the resistors on. Here is the finished setup:
Not my best wrapping! Thought I had ran out of the usual tape, and used the slightly furry stuff which doesn't look as neat, and then found the plain roll again. The panel for the resistors tucks around the side of the rad between the battery in the void behind the headlights, as my original plan of mounting the panel straight out clashed with the battery. This is it in the car:
The big downside with this is that you can't get the battery out without removing the fans as the plug is in the way... Unfortunately I had no time to prototype this so I had to live with it for the time being. I think it theoretically should be possible to slide the whole shroud out in one go to get the battery out but it's a pain, and I will need to try and remedy this longer term. In hindsight I should have kept the panel coming out perpendicularly but flipped so that the resistors are mounted in the fan slipstream; I may try and do this, although will have to try and find another Passat plug!
Anyway, I tested the three fan speeds with the battery prior to fitting and the all come on fine, and at different speeds (no reason why they shouldn't!) Since being on the car they do come on alright, although I can't really tell if all 3 speeds have been used yet. Not sure what temp speeds 2 and 3 come on and it's a bit hard to tell over the engine noise.
Don't know if it was worth the hassle and expense yet, but the fans seem a bit quieter and on the initial slow (20-30mph), albeit, short driving runs the temp seems to have stayed about 90-100 degrees.