2007 E61 530xd - help me diagnose a start-issue...pls!

Hi friends,

It's a bit longer one, but just because I've tried to narrow it down...

I’ve been dealing with a no-start issue for 2 weeks now 😄. It started 2 weeks ago when I was starting the car in the evening and noticed the starter really didn’t want to go and was turning over slowly—the next day, I didn’t start at all. At first, I heard a relay clicking rapidly during startup, and then nothing. My first thought was that the battery was dead…

After a few attempts with jumper cables + a booster pack—nothing.

The needles on the speedometer always jump up, I hear a click, the needles drop back down, and then silence.

I connected INPA and found there’s some issue with the starter itself/ignition signal (I think?). Live data showed that during startup, the starter actually tries to turn, but immediately cuts out (OFF -> ON -> OFF).

So, 2 weeks later, I replaced the starter (OEM Bosch) + some other things, and again—nothing. The starter is mounted correctly, the cables are attached properly, and all contacts are clean and tight (including the main positive cable from the battery and the engine ground). I measured 12V at the starter for both the main feed and the signal.

As I was constantly going to the back to disconnect the battery for the starter install… I noticed that the connector for the BST (Battery Safety Terminal/pyro-fuse) was disconnected.

And here’s the thing…

Since the car also won't let me shift out of Park (P)!!!, it occurred to me that this fuse simply went bad… when I inspected it, there was a lot of oxidation at the connector, and it looks like someone in the past probably tried to bypass this error with that cable.

After plugging it in, the "Restraint System" airbag error lit up, and still—nothing.

My theory is that the faulty pyro-fuse is preventing me from shifting out of Park and is also causing the issue where it rejects the hundreds of amps trying to pass through it 😄. Has anyone encountered this before, please? Otherwise, if anyone wants an original Bosch starter, I have an extra one now 😄 (the old one).