Long story short, I bought a '09 IS F from a friend back in the summer. Two weeks in, the brakes needed to be replaced. While it was down waiting on parts for the brakes, I decided to do the coolant valley reseal, valve cover gaskets, and spark plugs.What should have taken me a weekend or maybe a week turned into 2+ months due to parts getting lost in transit, reorders, other projects with strict deadlines, and time spent with the family. Followed many of the great writeups here to get the work done and used the service manual. Everything is back together except adding coolant (another item lost in transit) and header down pipes getting some attention from a welder friend after the PO put cats and flex pipes on the DPs but left some large holes in the weld.

I am hoping to have the car on the road this week, so I was tiding up and decided I wanted to get into the trunk to make sure I had stashed anything back there. I put the battery back in, pushed the interior trunk release, and nothing. Verified the battery has 12.3v across the posts and the wiring terminals. Even disconnected them to clear the contact areas. Reinstalled battery terminals. Same situation. I spent a few hours reading threads last night, but most of them seemed to deal with dead cell batteries or batteries getting too low while sitting in storage.

Here is what I know so far:
- Car recognizes fob when pressing unlock (beep and signals flash). Does not respond to lock function from fob. I did not try the trunk release from the fob.
- Car will go into ACC mode when sitting in car with fob and with card style key. I have not tried to start the car due to open headers and no coolant.
- Exterior lights do not respond to column combination switch
- Brake lights do not respond to brake pedal
- Locks, window, and mirror switches do not respond (driver's door or any other doors)
- Trunk release and gas door release do not respond
- Infotainment screen does not come on in ACC mode.

I checked all the fuses under the hood, and they were all good. I doubled checked I had all the grounds attached from when I did the coolant valley and valve covers. I do need to check the module under the battery. I recall trying to remove it to get more room while doing the valve covers, but then found the right way to hold my tongue and didn't have remove it. I remember installing the screws, but I am going to double check the connectors this evening. While I have the battery out, I'll check and clean the body side ground connection.

Other than pull the battery and try again, where should I focus my attention first? The car has headers, Varex exhaust, RR Racing intake, AOS, and RR Racing tune if that makes a difference.

 

Follow-up. It was a bad fusible link. The primary path was broken. I am not sure how it failed. The battery was hooked up correctly. I suspect I may have broken it when I was removing the power cable nut. It was very tough to remove, and I had to use what I felt like was an excessive amount of force to break it free. I got it swapped out last night, checked for power and started it up.

 

I was about to say, all the systems you listed that were malfunctioning must have had something in common. Looks like you found what it is.