Sourcing +12V signal when engine running without EK1 module?

Hi. I've built a 'powerwall' for my work van that embeds 4x 100Ah LiFePO4 cells, a RedArc charger, a BMS, and some outlets, into the wall panel cavity behind the driver's seat. It works great, and is almost ready to go, but I saved one bit for last... getting the signal for charge on/off based on vehicle ignition. I assumed that the interior fuse box would have some circuit I could tap off of, but it appears that entire box is switched on accessory. I couldn't find a reference on this forum for getting that signal without the EK1 upfitter module, which my truly base model van doesn't have.

Does anyone have wiring diagrams or reference material on how to grab that signal from a harness below the dash somewhere? Most of the connectors are a **** to probe, especially if they need to be connected for the van to start. But I'm sure one of those colored wires goes hot on ignition and I could solder a splice in, I just need to figure out which one.