Code U0155 on my 2006 Mazda3 S Grand Touring 2.3

3x Mazdas
I've had this car for 14 years and it's been relatively trouble free. This isn't major, but annoying. In the past 2-3 weeks I've been getting a Check Engine light on startup, but not every time. It's usually after the car has either sat overnight or for several days. I have cleared the U0155 code 2-3 times but it will come back. When I did a search for that code I've seen references to instrument cluster issues or possibly different computer modules not talking to each other properly. The only thing I've done to the car in the recent past is I changed the spark plugs about a month ago. I do believe I did that before this Check Engine Light problem started but it didn't start immediately after the plug change.

Just last night and again this morning I've noticed something: Even without the key turned on or in the ignition when I popped the truck I could see many of the warning lights on the dash lit up (but not fully lit - kind of dim). I have also noticed that the trunk light is fairly dim when I open the trunk - it's normally pretty bright now it's not bright enough to really see in the trunk if it's dark outside. I was assuming the bulb was about to go out or something. This morning as I was getting out of the car at work (still dark outside), after I killed the engine and removed the key I pulled the trunk release. The dash lights came on dimly. There's some corelation between the trunk being open (the trunk light on) and the dash warning lights coming on dimly. After I got out of the car and shut the door and closed the trunk, the dash warning lights went off.

Back when the car was new Mazda had to replace the junction box/computer/fuse box that's mounted on the firewall above the passenger footwell about 3 times over a few years. I think *I* figured out the problem with that was a possible leak at the base of the windshield causing corosion in the computer box. I read about the known possible leak by doing some internet searching. After the last time the dealership replaced that computer box I wrapped it in plastic - that was about 9 years ago so I assume I was correct and no more problems.

Otherwise, my car is running fine. Has anyone else had something similar happen? Thanks for any help!