Hi Matt,
I have a hardwired Valentine (thanks for your help waaay back when) which is tapped into a 12V accessory wire from the ignition bundle. The write up is at
http://www.4runners.org/articles/valentine/ It will only power when the ignition is switched on.
Also, I added a single, full-time 12V outlet like you are planning to do. That write up is at
http://www.4runners.org/articles/12v I used ASUDan's write up as a guide, but instead of using the type of outlet he used (lotsa glue), I bought one that can screw onto itself (a no glue install). Check out the write up to see what I'm talking about and try to find the same kind.
I also followed Corey's advice and added a 12V auxiliary fuse box inside the engine compartment. Read 'Step 6' in my Cobra CB write up at
http://www.4runners.org/articles/cobra . I bought the parts from Waytek but I've recently seen it sold at Pep Boys. If you haven't done it, its definitely worth doing b/c it really allows you to easily add power for future accessories.
As for draining out the battery, I really have no clue. I know you can buy a box that will switch off power by voltage and/or by some set time. See
http://www.4x4wire.com/toyota/4Runner/tech/chargeguard/
From personal experience, I've powered my mobile phone for 3 hours and when I came back, there was no problem starting the 4runner. I haven't left it on overnight so I can't say for 100% sure that it would drain the battery.
In anycase, the V1 and the Garmin are things that I NEVER leave out in the open so when I stop and park, those things get turned off and locked away.
Email or PM me if you have any questions.
Bob