Update, 3 weeksafter I reinstalled the ignition switch and ISM and the car worked;

1) I assumed that the issue had to do with either a connection in the ignition switch or ISM. The car started fine for 3 weeks with no signs of a weak connection or bad component. Weather has been low 30's F here for the past few weeks and the car still started with no issues.

2) Then just yesterday morning on my way to work, it wouldn't start. Same scenario, single click, headlights dim while i have the key in the ignition position, no crank, no start. After 3 tries, i pop the hood to take a look, close the hood, try again and it starts! Drive 25 miles to work, park it for 8 hours, go to leave and car does not start. I try about 20 times ad still no start! Got a jump from a friend and drove it home. Parked it and shut off engine, tried to restart and no crank.

3) Let it sit overnight, checked the battery next day and the voltage was around 12.6-12.7 (if you recall this is the new battery I installed) so I don't think its the battery.

4) removed ignition switch again to re-inspect. The switch has a manufacturing date of 2016 on it so that makes me think the previous owner just had a new switch installed. It looks brand new too so i'm not convinced the problem was the ignition switch anymore.

Back to square 1. I've already bench tested the starter (twice now) so I don't think that could be the problem. I recently started hearing some squeaky noises from the engine bay near the drive belts. these noises go away after driving for a bit. The only part I haven't checked/tested is the alternator. I just don't know how a bad alternator would make sense. Any other ideas?

Check the positive cable and the negative cable from the battery. The BST portion of the positive cable could be the culprit.

Ignition switch issues can be sporadic and symptoms are not consistent. I don't think you have eliminated the switch as the problem. Is the switch an OE switch?

On a lot of vehicles it's the ignition switch connector and not the switch itself that fails. Regardless, I think you need to learn about voltage drops and current so that you can pin down the problem when it's happening. I would say starting at the battery connections and working towards the starter.

The ignition switch is Valeo. I inspected the pins and the internal copper tabs and there is no corrosion.

I've tested the voltage drop at the switch. It has the same voltage reading at the switch as well as the battery terminals and at the jump start terminals under the hood. Also tested the voltage drop with the key at the ignition position. The voltage drops about 1 to 1.5 volts. I haven't tested the voltage drop at the starter with the key at ignition (mainly because you need to jack the car up and have another person at the key).

Is there anything else in the engine bay that I could test the voltage drop that could be causing a resistance in the circuit? Makes sense since jump starting the car works.

Don't forget about the ground side of the circuit.

As I read your posts you have not replaced the ignition switch.

My understanding is that you removed the ignition switch and checked it out. After you reinstalled it you had no problem for 3 weeks. You now have the same problem. IMO it is the ignition switch.

It is very difficult to determine if an ignition switch is working by visual examination. The points can look fine and the switch is still faulty. Certain points have to close or be open depending on the key position. I would spend the $100 for a genuine BMW ignition switch.

Yep I have a new ignition switch arriving today (i'll install tonight and give you and update). Part numner 61-32-6-901-962, Genuine BMW, with shipping and tax it was $80 from BMW Parts Wholesale. Fairly inexpensive to rule out that component.

The Ign SW replacement is an easy fix and a multi issue generator when they go semi-dead. I replaced one after getting radio and instrumentation issues that were random just by turning or wiggling the key cylinder in the column. 1 hr later all problem would be gone. Sometimes I think these SW go bad over time regardless of key cycle starts due to the vehicle just being energized when parked. There is 12 volts going through the switch 100% of the time even in "sleep mode", if I understand our Bus systems correctly. I am thinking of switching to battery disconnects on vehicles parked longer than one week of non use or storage. Yes, I realize I will loose the radio and HVAC settings unless I use a remote 9volt battery sourced connection through one of the aux ports (cig lighter for example).

New ignition switch installed but still no start.

I plan to replace the drive/ac belts and pulleys in the next week. When I have those out, i'll inspect/test the alternator too.