OK, this is an odd one - interior odor only when braking downhill

The brakes were done well before the smell appeared.

A tire contacting a fender liner seems possible, and one of the things I'll be looking for. I believe the smell started roughly around the time I mounted the snow tires, so I will be checking for any points of contact. That said, there is no audible noise that would point to rubbing.

I'm checking the whole front end over. The exhaust is on the right side, so if it's something hitting that it will be over there.

The smell appears and disappears quickly, and the intensity seems related to forces involved in braking and turning. Stay tuned.

Well, I'm stumped. Pulled both front wheels and the undertray. The tires are not rubbing anywhere. Brakes are not binding, and pads, rotors, and brake dust accumulation look similar on both sides. There are two leaks evident - the PS pump seal and the oil pan gasket - but neither is bad enough to be causing appreciable fluid loss, and neither is close enough to the exhaust to cause the fluid to come in contact, unless BMW's fluids are made from the same stuff as the JFK "magic bullet".

Unfortunately, I did discover that both front Bilsteins are weeping at the seals and have significant rust at the bottoms, and one piston has visible pitting in the chrome. Going to have to contact Bilstein about this, but I don't think it's related to the smell (I sniffed everything closely and found nothing that smelled similar).

The immediacy with which the odor comes and goes is making me lean toward something electrical, but I have no idea what that might be. I can't find any sign of melting wires or similar. Both recalls that had to do with wiring have been done (PCV heater and blower fan).

As much as I hate to do it, I may have to bring it to my indy and see if he can track it down.

Brake light bulb/housing, and you only notice it when you are on the brakes for more than a few seconds?

In reply to Knurled. :

I had already looked at the brake light housings, which appeared normal and unmarked. I did not open them, however. FWIW, these are factory LED taillights. Holding the brakes on (when waiting at a light, for example) does not produce any smell, regardless of how long they are held on.

Turn on the ignition, prop something on the brake pedal and see if the odor is apparent with the vehicle sitting. It might confirm that it's electrical in nature.

Edit: Never mind, it sounds like you basically covered that.