Ok, the mod worked fine in my 813 and 815 radio's. However, when I tried it on my 816 it does not work.

I did ground the wire to the chasy.

I also still have the problem with my droid switching the sound to the phone speaker when music is paused or there is to much time between sound. It's like it gets so quiet that the phone things the signal is lost so it transfers the sound to the speaker and the plug has to be plugged into the adapter to reset it.

 

The 816 was soldered to the top board like in the picture below. The board was a little different so the ground was in the pin just above the joint in the photo. You really need a ohm meter to do this mod. The ground can be soldered to the chasy. The volume was on the loud side so I had to set my player to about 40% volume to be as level as the radio and cd player.



As for my droid cutting off, well, it cuts off and back on as the music is about to play again, on the SC-816. If there is a 5 second pause in sound, I hear a low freq pop as my droid cuts sound from the cord, then another pop as it turns it back on. Kinda sucks but if music doesn't pause it I don't hear the pops. Quality is worth it.

 

Here are my instructions for the MOD I did to get the mp3 connection.
Radio mp3 Modification for the Volvo SC-813, SC-815 and SC-816 Car Stereo