I have seen quite a few methods online for doing this (not on a 135 though), BMW makes it extremely difficult to get to the fuse panel (in comparison to other cars) so I figured I would document the procedure to save some other folks some time.
I am locating the V1 at the top of the windshield and running the power from the fuse panel. The V1 will be on the accessory fuse and will behave as if it were plugged in to the cigarette lighter, without the messy looking wires.
First we need to get to the fuse panel it is BEHIND the glove box, FYI: if you see bitched up plastic, that was Fields BMW of Winter Park, not me.
Pull the little center thing out:
You should have all seen these guys before:
Then unscrew the 2 screws holding up the panel blocking access to the glove box screws, after you have that out the panel should drop down, be careful, there is still a wire connected for the light, disconnect this by pressing a small screw driver into the release tab.
With that panel out of the way you should have plenty of access to the glove box screws:
Unscrew all four of these.
Inside the glove box you will need to detach the gas (?) shock / soft opener:
Then press the tabs to allow the glove box to come past its operating range, there is one on each side:
Once both sides are free the glove box will come out easily.
Now we finally get some info on the fuse panel in this little card hidden behind the glove box:
Remove that for light reading, you will refer to it to locate the correct fuse for installation:
Here is a view of the dash with not glove box:
I am using fuse 08, the accessory fuse, and tapping into it with one of these guys, available at an auto parts store near you:
No easily available Ground in the fuse panel that I saw, so I used a body ground which is available via a piece of metal, unscrew and sandwich the terminal under the screw:
I ran my power wire for the V1 out the side here:
then up the A pillar, across the windshield and down the rear-view mirror, all under the weatherstrip or behind the headliner:
Here is a page with more pics
, including the shots of the fuse panel info card.
Now put everything back together, don't forget to reconnect the little footwell light, and tie those wires up neatly so they don't rattle around.
As always, if you don't understand, ask questions before you attempt this, you could probably manage to cause damage to you and your car even if you execute this perfectly, so do this at your own risk. Sorry to cut it short, running out for a graduation dinner for a friend.
UPDATE: One member has suggested you do not need to remove the glove box, just drop it (see below). Other have suggested it is easier to do the sunroof wire. As stated there are many methods of doing this, but the 135 fuse panel version (which is how I wanted to install, for reasons stated in posts below) did not have a DIY, so I figured I would document for the community.
Additionally, I don't know what the behavior of the detector is on the sunroof power, with the above method the detector behaves as it would on the cigarette lighter, which is, when the ignition is off, the detector is off, that makes most sense to me. I would presume the sunroof power is available if accessory power is on but ignition is off, which is not optimal operation IMO, but a small nuisance at most.