Yesterday I did a coolant replacement on my '99 528iT. I had no problem getting the engine drain plug out, but had a time with the radiator plug. The previous owner told me it had a replacement radiator/pump/thermostat (sound familiar?), so this may help someone out in the future.
The radiator did not have the same drain plug as shown in the Bentley manual (OEM, obviously), but rather a blue tab like plug. This plug only turns 90 degees and stops. It took me longer than I'd like to admit to figure out that it simply pulls out - rather than unscrews.
I searched here and at a couple of other forums, but found no mention of this type of plug. Perhaps this may be helpful to someone else - someday.
BTW. 50/50 BMW coolant/distilled H2O and a bottle of Water Wetter for good measure :thumbup: