On the GS this is per front door. The rear doors are wired differently.
I worked with member itsRaymond on this and we actually discovered the Door ECU varies the voltage on the Pink wire for the front door (not the blue...hence why it's always good to double check).
So we connected the Pink wire to an Ignition On 12V source within the door to have the lights at full brightness at all times when the vehicle in On.
It was actually super easy to wire up as everything was done inside the door.
You simply repeat the process for the front passenger door.
If you want to do the rear doors then it's a completely different process as the rear doors do not have an ECU inside them that controls the lights...it's a more simple circuit that just requires you to Ground whatever light you want to be on...whether it's the ambient light, door handle light, or foot light. Each light will have a Power and Ground wire...simply cut the Ground wire and run it to a Ground point within the door...anywhere there is bolt should be sufficient.

 

Hi Sasnuke

I am also trying to re-wire my 2018 GS350 ambient lighting to always be at full brightness while the engine is on.

I have just been reading through the thread

So it looks like
For the front ambient lighting
I have to dismantle the door's interior to get to the door ecu
From there I have to locate the pink wire for Q29 and splice it to an ignition on source. Could you please identify which wires these would be?
For the rear door I just locate the brown wire (ground) to the ambient lighting assembly and connect to a bolt?

Thank you very much for your help

 

There is only one ignition wire in the door, and it's actually in the diagram in the posts above. I'ts the Black wire at the top left of the diagram...it goes to the ECU as pin 3 (SIG wire).
You have to cut the Pink wire and splice it onto the Black wire...do not cut the Black wire. The end of the Pink wire that goes to the ECU needs to be capped off, or taped up, since it will be live.

For the rear door that is correct...the wire is actually Beige (BE). Cut the wire and find a bolt to Ground it to...something that bolts to the frame of the door preferably.
Any cut ends of any wires that are not being connected to anything should be taped off.

 

Hi Sasnuke

I have done the rear doors and am very happy with the results

I have tried the front door by cutting the pink wire and splicing onto black
It stops it dimming when going into drive but the lights dont turn on when I unlock the car
I have then reconnected the pink and black in parallel and that fixed it. Is this safe?

Thanks!

 

It's hard to say what will happen with the new parallel circuit without being able to see what's going on insider the ECU...via a diagram.
Once you cut the pink wire the ECU lost the ability to control the light, so yes it wouldn't light up when you unlock the door.
When the vehicle is running the ECU would reduce the power to the light to around 10V...hence why it dimmed.
Now that the Black wire is connected in parallel, the light is getting 12V, but so is the ECU terminal that would now be about 10V.

I was thinking maybe a diode installed in the pink wire between the ECU and where the Black wire is spliced onto it...with the current flow of the diode going from the ECU to the light.
It also might be completely fine the way you have it now.