2003 Prius multiple electrical issues

I recently bought a 2003 Prius that hasn't been running over a year. The seller claimed that it only needed a traction battery. I already have a 2002 Prius so I thought I can either fix it up or use it for parts. The car only cost $350. If it's not worth repairing, I will strip it for parts and junk it.

The old traction battery had lots of bad modules, so I removed the good one from my 02 Prius and dropped it into the 03 with the naive notion that it would just start up and run. As it turned out, it wasn't that simple. Here is a list of problems I noticed so far:

-when I plugged in the aux battery (the good one from the 02 Prius), it immediately started honking. After fiddling around with the battery enough times, it eventually stopped (I'm assuming this has something to do with the immobilizer)
-When I turn the ignition switch to the "on" position, the dash lights don't work but when I go to start, they come on
-The MFD does light up or turn on
-The alarm light does not come on
-No power to the HVAC controls
-When I go and turn the ignition switch to on, the dash lights come on and the engine starts
-The triangle of death is displayed when the dash lights are on and the engine is running
-When I put the car in gear (reverse or drive), car does not move but I feel a slight jolt in reverse
-Car shuts down at random, engine will not say running
-Tried to run a code reader. Car does not power on long enough to get a good reading but I did get a 3125 code.

Not sure if I want to trust the codes at this point. I returned the traction battery to my 02 Prius and noticed that the battery was drained and had the "turtle light" for the first few minutes of driving. I did test the 02 Prius to see what powers up and what doesn't power up with just the 12v battery and the traction battery disconnected. As it turns out, everything on the dash should illuminate and power up. At least I have something to go by now without having to keep installing the traction battery. My goal now is to check all the relays, fuses and connectors and see if I can get everything powered up with the 12v. I do believe the 12v battery is good, at 12.5v disconnected. The 02 Prius runs just fine with it.

My question is, if there is anyone still reading this, any suggestions on what to check first? I heard there is a body ECU somewhere in the car. Would that be a good place to start? I plan on working on it this weekend and updating this thread.

 

I have verified with a volt meter that there is no power going to the "PWR 1" fuse. This fuse is located underneath the dash and is responsible for supplying power to other systems such as the dashboard and MFD. When I attached a jumper wire to the PWR 1 socket, everything powers up normally. Not sure why it's not getting power. The wiring diagrams I found a Google search are crap.