I dunno about your year but i rebuild mine. Has been good. But i can buy the whole regulator from BMW for my car.

Thankfully mine was warranty covered. I believe the unit was $800 and labor was about $600. Total was ballpark 1400. I payed about $100 for some reason, probably to confirm it was a
faulty alternator. Good deal for me, best of luck to you.

EDIT: These are dealer prices.

Sounds like yours is definitely an alternator related issue with the dimming instrument panel light. Mine was a contaminated wiring harness connector at the transmission. The factory had not packed any sealing grease in the harness plug. After a trip through the car wash one day, the transmission went into fail-safe. When it dried out about an hour later, it was okay. Before tracing the problem, it happened again after a heavy rainstorm 400 miles from home, only then I couldn't get the lever out of neutral for two days! Once back home, the mechanic spent an hour and a half cleaning connector pins - it was full of dirt.

2002 X5 3.0 241,000 miles
2004 325i 107,000 miles

8 cylinder X5 E53 2003 or earlier have water cooled alternator.

At what RPM does your X idle at operating temp?

Most purchase a rebuilt alternator for less than $100 more than the rebuild kit. There are several on Ebay. Choose one with great ratings if you find out it is actually is the alternator. Certainly verify if either the battery or alternator or bad prior to purchasing either . The battery test should be a load test which can be done for free at many auto part stores. The cluster info is fine to check alternator output.

Bear in mind that It is possible you have two separate issues or the charging system could be working OK.

Well, I checked the voltage through the cluster unlock. Interestingly enough, its charging at 13.3-13.5, with all accessories on. Seat heaters, A/C, radio etc....

The service engine soon light is still on, but no more Trans Failsafe warning.... and it drives fine. Like nothing happened!

What do I do next? Would there be code(s) if I run a OBD scan?

Have you load tested the battery?

I would monitor the voltage as sometimes an alternator problem can come and go.

With the service engine soon light on it is likely there will be a code(s) but it is not a certainly especially when using an OBD scanner. I would certainly check that, read and then clear any error and see if it comes back.

That's my next step. I'm going to roll down to my local battery place and have it tested.

Then I'll scan it. My buddy has a scanner.

Could the Trans Failsafe have anything to do with driving in rainy weather? My old E36 would do the same thing if it was rainy or wet out, but if you just turned it off for a minute, it would restart and be fine. This X5 didn't clear the Failsafe code until days later.

Yes, there are trans related electrical connections that could be damaged or are dry (no electrical grease) that are allowing water in. And, it can take days before the connection drys out.

Makes sense. I think I'm going to scan it, then throw it up on my buddy's lift, and check all the connections.

Good luck!