look at this.
Splicing In New Resistor On Range Rover Classic
and this.
http://www.pelicanparts.com/944/electrical/944_84_AC_2.jpg
do you see how the resistance is daisy chained on the grounding side of the motor? the fan gets battery voltage and the switch varies the current level by the resistors in series.
I get..
1.) brown(stock 924) to brown(rover)
2.)br/wt(stock 924) to red(rover) this is speed 2----0.5 ohms
3.)br/ye(stock 924) to blue and one black(rover) this is speed 1----1.5 ohms
4.)br/bl(stock 924) to the other black(rover) this is speed 0----3.5 ohms
high fan speed bypasses the resistor pack and goes straight to the fan switch with no resistance to ground/earth. .
by connecting the on black lead to the blue (rover) you are effectively daisy chaining the resistors like the stock windings. if low is too low, compared to stock, move the black wire that you connected to the blue to the brown wire on the rover part to get only 2.0 ohms...which is probably closer to the stock resistance..check that.
clear as mud?