Thank you for this thread! Hardwiring my trusty V1 was the first mod I did to my new car and this gave me the courage to do it.
I want to address a few issues I found that might help others doing this... I think the two hardest parts of this process include: 1) removing the dome light assembly / dropping the headliner and 2) finding a fuse to tap. Do all this at your own risk.
Before anyone proceeds with the install, start with a good phone cord... preferably the one that comes with the v1. I tried 3 different phone cords that didn't work right before I broke down and ordered another one from v1 (luckily I bench tested them before trying to install them). Note that Valentine Research has free shipping on their parts so my advice is just order one if you don't already have it (or used it on a previous project).
1) Remove the dome light and drop the headliner
Both are very easy steps once you know how to do them but (atleast to me) were daunting tasks. Lowering the headliner has already been covered countless times throughout the forum... it entails just removing two screws. At that point the headliner drops about an inch and a half without any further struggle with clips or anything else.
Once the headliner is dropped you have access to the two front tabs on the dome light. This thread contains a few photos that will guide you to exactly where the tabs are:
http://www.e90post.com/forums/showthread.php?t=273231. Just depress those tabs and the assembly can be removed.
To ensure that the phone cord doesn't get ripped out or pulled out by accident I zip tied it to a large part of the wiring harness for the dome light assembly.
2) Finding a fuse.
I used a small blade fuse tap as everyone else has and I found an empty fuse slot. I think I used fuse 10... it was something related to power seats (which I don't have) and it's switched power. Unless you have a fully optioned car you should definitely be able to find a few empty fuse slots and not even have to deal with actually having a "pass through" fuse in your add a fuse.
As to what amp fuse to use, I also ordered two extra 2 amp small blade fuses from Valentine... the same ones that are used in the cigarette lighter power adapter for the v1. I figured if it works there it should work in the car.
Hope that helps someone!