350sdl acts like no battery is installed

[upshift]

Battery died in my 350sdl- dead cell - I bought a new one, started the car, drove around for 15 minutes or so.... headlights worked, radio worked, everything was good.

Came home, shut off the car--- nothing. No lights, no crank, no dash lights... nothing.

The battery is pushing voltage to the starter, but absolutely nothing works. It's like there's no battery installed in the car.

I tried hooking up another decent battery with jumper cables but it made no difference.

I installed a brand new ground strap a month or two ago.

What on earth is this? I can't find a wiring diagram for the gen2 126 so I'm a bit lost.