fishtrap, did you ever figure out this issue? I have a similar problem with my 12v that has been going on for quite some time, and I cannot nail down the source of the problem. Originally, when I had this issue, I found the connection from the IMA to the DC/DC converter to be loose. Tightened it up and drove 6 months without issue. Now, it's reappeared and that fix isn't working.
Essentially, I'm having similar problems as you with the brake and battery lights slowly flickering on (the rest of my dash works, though, unlike yours). I've resnugged all the grounds (they are 6 months old, replaced those last time, but I should probably clean the contact surfaces), checked the fuses, disconnected the IMA system properly even though it was working, but the issue persists, and I cannot pin down its source yet.
Just wondering if you had any such luck? I'm thinking it has to be a short somewhere or the DC/DC converter is dying because the battery and brake lights aren't consistently on. Instead, they slowly flicker on when the 12v is near death. For instance, during my commute to work this morning, they started to flicker on, then became solid, so I turned off my headlights, they went off, and I was able to get to work with the lights only faintly flickering on as I turned hard into my parking spot at low speed (surely EPS causing the flicker on a low battery).
Thanks for any help and hope you have figured it out! I'm sick of carrying two 12v batteries in the car