Next rain comes and sure enough it smelled like mildew again. So I bought some HELP! speedo cable (per Bentley) and cleaned the front drains out. Not much there actually.
Then followed the tech article (linky) to clean the rear drains, except I did a lot more work than was necessary. You need to remove the rear bumper to get to the rear sedan drains located in this picture where the round plug is:
Well it's clearly not there on the avant. After STFA, somebody noted it was behind the rear wheel liner at the top. So I removed the liners (I hate doing this for some reason) and you can barely see them. See below.
For some reason, this drain is different than the fronts. These have a nipple that makes it difficult for the water to drain. I squeezed the nipple a few times and water started to pour out. So I took some dikes and snipped the nipple off so that water can freely run out in the future. Problem solved. BTW, I couldn't get my speedo cable to go all the way up to the sunroof, it stops at the bend at the roof.