I finally managed to get round to getting this done. I've taken a slightly different approach to some of the ideas on this forum, but it's basically the same. Here's a wiring diagram of what I did.
A quick overview. This mod is for standard vehicles that still have functional heater plug solenoids. It's simple quick and doesn't require that you have high amperage cables permanently live. So it's a bit safer too. The current is via an ignition fed live feed. I've also utilised the redundant glow plug warning light in the cluster, which could be useful.
Placing the switch. You need to establish where you want your switch. I've located mine just above the ignition switch.
Routing the cable. You will have to route the cable through the firewall. The best way I found to do this was to remove the second battery. You obviously don't need to do this if you've only one battery. Incidentally, disconnect the other battery as well. This wil prevent stray sparks.
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From inside the vehicle, remove the heater duct from under the lower dash on the passenger side. Remove the radio surround, the drivers side lower dash, instrument cowl and instrument cluster. Locate the wiring grommet that's situated on the passenger side. Top left of the footwell just above where the carpet stops. Feed the cable through here. It will come out in the engine bay, just by the second battery tray.
Pull through enough cable to safely route round the battery to the solenoid. Back inside the vehicle. Route the cable under the dash, behind the centre console and through to the hole for the switch.
Connections. Next. You need to find an ignition fed live source. In this instance I've used the feed to the rear window demister. (Blue cable with red tracer) and scotchloked the feed to the switch into that. Connect this cable to one side of the switch
Connect the cable from the engine bay to the other side of the switch and secure the switch into position. I've put an inline fuse between the switch and the solenoid. I've located this behind the centre console part of the dash. If it blows you only have to undo 2 screws instead of four. I've used a 10 AMP fuse as this system should only require 2 - 3 AMPS current.
Back under the bonnet. Remove the original spade connector to the main solenoid (Black rubber caps) insulate it and secure out of harms way. Then simply put the new wire onto the spade connector.
Glow plug warning lamp. If you want to utilise the glow plug. Then do the following - On the back of the instrument cluster. locate the vacant glow plug location.
Find a bulb holder that will fit or as in my case nearly fits and if necessary enlarge the hole slightly. I used scotchloks and simply connected one side of the lamp to the cable from the switch and the other side to a suitable earth. Check to make sure the lamp lights when you push the switch with the ignition on and hey presto jobs a good un.
Finally. Having tested the switch to make sure it all works fit everything back together.
Jobs done.
*EDIT*
This is particularly useful. I've piggy backed the overide cable with the ECU cable onto the solenoid. I asked an Auto elec friend to check it out before doing it and he's happy that it will work without damaging the ECU. I've been running it like this for a week to make sure and I've had no detrimental effects.
It also has the added bonus, if you've added an indicator lamp, of letting you see when the plugs are heating both pre heat and post heat cycling.
Parts used. 16 AMP Switch.
Inline fuse (10 AMP)
3 x Scotchloks.
7 x Spade connectors.
3m 16AMP cable.
12v Bulb holder and 3W bulb.
Pajero UK
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another well detailed thread 
...ive been having trouble getting mine started(don't have glow plug light on dash) but still do the 5 sec rule, been told that it could be the glow plug relays... could i do this mod?? when the paj does finally start, throughout the day its brilliant..starts first time...(typically i see this thread after ive just ordered some new glow plugs lol) 
