It's well known that there's a big volt drop from front to back on vehicles with rear mounted batteries such as my Diesel, this causes dull headlights, slow wipers and slow fans.
The fuse box looks like it is intended to be fitted to other vehicles, such as those with front mounted engines (Golf etc), and as such there's a spare position (2) in the fusebox for a relay which would feed the glow plugs in these cars. The terminal on the back is marked 'N' on the diagrams (Thanks Andy Syncrosport, and Itchyfeet) and is a large (9.5mm) spade connection.
Instead of feeding power outwards, this terminal can be used to feed power BACK IN to the fusebox.
You can connect a heavy gauge wire from this terminal, and link it to the high current feed for the high-speed setting of the radiator fan. After this, install a high-current relay (such as a number 100) in position 2 then you've added a second current path to the fuse box which kicks in when the ignition is turned on.
It does NOT defeat the function of relay 8, which disconnects loads during cranking, it simply reduces the effective resistance of the cabling bringing power from the back to the front.
In practice, it's only a couple of hundred millivolts, in my example the voltage seen on an ignition feed at the back of the fuse box goes from 13.15-13.2V (fluctuating) to 13.38V. It doesn't sound like a lot, but there was a noticeable difference in the brightness of the headlamps when plugging and unplugging the relay in position 2.
It would probably be a good idea to add a fuse in the thick wire, or you could connect to the relay side of the high-speed fan fuse, which would provide adequate protection. The fuse and the cable is very over-specced, so is more than capable of feeding a few amps into the fusebox as well as running the fan.
Relay out...
Relay in...
Here's the heavy terminal...
Number 100 Relay in position 2...
Yellow Terminal on Fan Feed...
Spade terminal plugged in...
