I wired in my 135 like this with no issues. Took it out and moved it to my '08 M3 and can't get it to work. Followed the DIY and it will turn on, but then it just "hums". As soon as I plug it into the cigarette lighter adapter it works perfect. Plug it into the hardwired and it powers on them just emits a solid hum noise. I grabbed a standard phone cable from Target/Walmart - could that be the issue??
It's what I used on the 135 as well and it was OK.
Any suggestions appreciated!