It's relatively straightforward to keep a neat, factory-looking fog light on the Crown S170-series as the rear light cluster bulbs (all 4) are the same dual-filament T20 type.
As standard, the rear lights operate as follows:
I have used the outer lower bulbs as fog lights. The 2nd filament is the fog, whilst the primary filament is the rear running light.
(I wanted to use the inner-most bulbs but that would mean more work running wires through more interior trim pieces and that is too much like hard work right now)
The hardest part about the whole process was removing and replacing the light clusters from the car body as the space is VERY tight to get your hands in. You need slender fingers, a small 10mm socket and ideally a 10mm spanner with a flexible ratchet head just to make things a little easier.
You will drop the nuts when doing this but they typically tend to fall through to the boot floor so can be retrieved without too much issue.
Remove the light cluster from the car body:
1x 10mm nut securing the lower trim piece below the light cluster. The trim clicks into place with a fastener on one side but may need hep by wedging with a plastic tool (so you don't scratch the bodywork).
You must remove this piece before you can remove the light cluster fully.
3x 10mm nuts holding the unit to the car. 2 are easy to get to but the 3rd is practically inaccessible. Loosen it with a small 10mm spanner and then undo it with your fingertips (I have delicate IT geek fingers and could only just get a touch on it). You may be able to fish it out by hand but if you drop it, it should be retrievable from the boot floor.
Once removed, disassemble the light cluster so you can get to the lower bulb and the look connecting to it.
Strip back the sheath on the wire of the lower bulb holder and save for later. This was white on the passenger side and black on the driver's side for some reason - not sure if relevant.
With the wires uncovered I found that the 2nd filament wire (GREEN + WHITE stripe) was not connected to any source. The wire was taped up in the loom. This was the same on both sides of the car.
Really handy though as Toyota have left me a wire to connect my fog light live wire to!
For reference, the wire colours are as follows:
Solid GREEN = live for running lights - the primary filament
GREEN with WHITE STRIPE = live for 2nd filament
WHITE with BLACK STRIPE = earth
I removed the black tape and crimped a spade connector to the GREEN + WHITE STRIPE wire - this will be the live for the fog light fog.
The bulb is already earthed so no need to wire anything for that.
It's a good idea at this point before refitting the light cluster to replace all the bulbs as they're tricky to get to when it's all back in the car. I also didn't refit anything until the complete circuit was made up and tested, just in case there were issues that needed troubleshooting.
To pass an MOT in the UK, the switch for the fog lights has to be mounted on the dashboard where the driver can reach and see it during driving and it must be illuminated to show the fog lights are on (or have a bulb in the dash cluster).
3-pin illuminated rocker switches are dirt-cheap and don't take up much space so ideal for quick MOT fixes.
These switches need a positive voltage source and an earth. The 3rd pin is the source for the fog light bulb.
For the positive, I used a multi-meter to probe the wires from the ignition barrel to find a 12v source that was live only once the ignition was on - this was so I could ensure I wasn't going to connect to a permanent live feed and risk creating a drain on the battery.
I got lucky here as my Crown has an aftermarket remote engine start module which has been spliced into the ignition circuit using a harness to interrupt the factory wiring. I've used this to link into so I don't damage any Toyota factory wiring.
The wire I used was blue in this instance but the pin is the far left in the ignition barrel connector. This wire supplies 12v on the first click when you turn the key (position ACC).
For good measure, this positive source also has an inline 10amp fuse in place.
For the earth, I've run a wire from a mounting nut/bolt under the dash that is part of the metal bodywork.
There are no switch blanks in the crown to modify to fit the small rocker switch unfortunately so I have installed it directly into the dash on the same piece of trim that the wing mirror controls and ignition key are located.
Incidentally, this is the exact location of the rear fog light switch from the "winter comfort" factory option and as such, the dash piece has the moulding on the rear for the fitting of the Toyota switch.
I've drilled out this hole and used a dremel to further widen to fit the circular shape of the rocker switch.
You can see in this picture how the rocker switch (the 3 pins) fits snugly into the existing hole where Toyota switch would go (if it had been spec'd from new).
All that's left is to run a single long wire from under the dash, along the driver's side floor (tucking under trim pieces), through and behind the rear seats to the rear light cluster. Here I split the wire off and ran it along the rear of the boot space to the passenger-side light cluster so both bulbs could be activated from the same switch/wire. Terminated with spade connectors they were tucked under trim and connected to the refitted light clusters/bulbs.
The 3 wires can be fed up through the dash to the opening near the ignition and connected to the rocker switch. The live for the fog light bulb is the middle pin.
When the switch is thrown, the 2nd filament in the lower outer bulbs of the rear light cluster is switched on and the rocker switch in the dash has an LED to show the circuit is active.