I have just finished doing this mod on mine, Only I have put the relays above the fuse box so there's less wire for the power to travel down (and harder for to find) & wired my kill switch back in.
Now when I stand on the brakes the dash lights don't dim.
mat_fenwick wrote: hidden switch into the earth wires
I have had 'interesting' (to say the least) experiences when trying to cut multiple neutrals with 1 switch. It tends to back feed & switch a relay (or 2) on & uses the others as a earth. I initially tried wiring the headlights & fog lights in this arrangement. Then when I turned the key off, the headlights would go off like I intended, but the fog lights would then come on!
1992 Citroën BX 1.9 Diesel Meteor 4x4 - The Project.
1992 Citroën BX 1.9 TXD (with GTI engine; Mulleys old car) - Parts car.
2004 Citroën Xsara Desire. (Now gone).
2016 Ford Focus Zetec - Daily Driver. (Absolute bone shaker).
What I'll add to this now is that around a month or so I suffered a repeat of the failure to crank, solved by a short length of wire direct to the start solenoid trigger terminal. It turned out that the contacts in the (cheap ) relays I had used had become high resistance with only a short period of use. Nowhere near their rated current capacity either.
i always use old rellays from off cars.. toyota relays are really good and though say 40 amp, will survive a constant 80 amps.. with a little voltage drop.. but always make a good connection.
i did this on my panda for the lighting ring. then i just added every other relay to that haha. now i have a 150 amp switched and fused "ignition live". for running my other car's spot lights and ham radio gear.
(just over 1kw of power) "pull out on me will you!"
to find) & wired my kill switch back in. 
) relays I had used had become high resistance with only a short period of use. Nowhere near their rated current capacity either.
