Inverter or Not

sounds like there's a problem with the wire running from the 12v os to the front. all the connections good at the battery?

 

First I would verify the fusible link in the (+) terminal connector is intact and make certain all the connections are tight.
Are you near Columbia?

 

I always thought the 12v connection to the small battery was weird how it had multiple harness connection to power the car. Since you replaced the traction battery, sounds like you know your way around, but are one or both of the harness connections at the positive terminal seated well?

 

Just one to power the car, the fat one. The skinny one is just a voltage-sensing wire so the DC/DC converter up front knows the actual voltage at the battery. That's what the 5 amp fuse is for.

 

Just to rule out some peculiar form of 12V battery failure, haul the battery around to the front and use jumper cables to attach it to the post in the fuse box and some convenient paint free bolt. Whichever one you used for the jump pack. Assuming it starts, what is the voltage at the positive terminal? Is it the same voltage you get with the jump pack? If it doesn't start, well, very questionable battery. If it works, the battery is good.

If all that looks good then ....

As others have stated, it is probably an issue on the thick cable (including connectors at both ends) which runs from the 12V to the fuse box, or it is a ground problem (even though you say the ground is good). Or maybe there was some coating on the posts from the factory which is creating a high resistance connection to the car's post clamps? Jumper cable clamps could cut through that while cylindrical clamps might not. What about the grounds at the front of the car, are those all good?

It sounds like you know what you are doing, but you need to systematically rule out every single junction and conductor in the path to and from the battery, as any of them could be at fault. Because it is easy, I would unbolt the ground cable at the fender, clean both sides of the cable, and bolt it back on. Use a post cleaner on the battery and the clamp cleaner on the inside of the clamps. If it still doesn't work hook two jumper cables (red side only) end to end and see if running that from the positive post to the jump post lets the car work. Or just use one if you happen to have one long enough to reach. That might work if the issue was corrosion at the fuse box end of the long cable, under the nut (somewhere) on the retaining post. For the ultra strange I suppose it is possible that the path from the ground near the battery has unusually high resistance to the front of the car. Typically that would happen with corrosion where the ground cables attach to the body. If the car had been reassembled from two vehicles, would the welds be high enough resistance to do this? I wouldn't think so, but while I have heard of such cars, I have never had my hands on one. (I really hope that is not the problem!)