He has a link to a forum for the fix for Windows users.
aztartarcontrol suggested a fix:
After a little bit of fiddling with my 3GS with iOS4, I’m pleased to see it working with my Audio20.
These are the steps that let me have it fully working.
1) Install iOS4
3) Update all the existing applications
2) Sync a zillion time with iTunes
3) Perform a full backup
4) PERFORM A HARD RESET (Home button + Lock button til you see the Apple logo)
5) Go to your car
6) Delete every paired phone from your Audio20, all the contacts, everything from pre-existing pairing.
7) Start pairing the bluetooth only. Verify that calling works.
8) Now sync your contacts and verify they’re all transferred correctly.
9) Now switch off your car, cycle your bluetooth (turn off – turn on).
10) Now go back into your car and see if it pairs automatically. IF IT DOESN’T THE FIRST TIME, JUST REPEAT STEP 9-10. (First time after pairing mine wasn’t hooking up the BT signal at all, just did it all again and worked.)
ONLY FOR PEOPLE WHO HAVE AN IPHONE MEDIA INTERFACE
11) Now plug in your cradle to your iPhone.
12) Verify it plays music properly, it lets you skip tracks, change folder, fast forward and rewind.
13) Try to place a call while listening to music.
14) Try receiving a call.
15) IT CAN HAPPEN THAT IN ORDER FOR YOUR BT TO WORK, YOU HAVE TO FIRST LET IT PAIR AND ONLY AFTER PLUG IT IN.
16) Now simulate normal behaviour (as before the upgrade) and check if there’s any issue.
17) If everything works, enjoy your iOS4 in your shiny Mercedes.
Hope this helps.