mike552, since I want the lights to only come on when the door is open, I my knee-jerk reaction was to assume that I should just tap into the existing light doors. I only brought up the BIU because I had seen it utilized in other threads. I have no doubt that it can be used (maybe by someone that has a tad bit more experience), but I am coming to the conclusion that might not be the best way to go. Even though I've never had the door panels off on this Subie, Ive had them off in all of my other cars, so I have no aversion to taking the doors off. But I like your straightforward idea of testing to see which wire is the door light and tapping in to that source under the dash. "The big deal" is the fact that I've just neveer done this stuff before, haha. Seriously, I'm a noob when it comes to electrical. Mechanical work I'm ok with. Electrical and body, get me a doctor, haha. Thanks for your reply.
plain OM, So what I'm hearing is that I can just tap into the dome source or door source? I want the light to stay on as long as the door is open, so the door (always-on 12-v) source is the winner, it sounds like. I am thinking people in other threads (adding footwell strip lighting to their Subies) were using one of the sources flowing into the BIU, such as the door or dome light wires. Your response was very informative, thanks!
grossgary, again - I'm new to this stuff, so I've never used a relay. From my limited knowledge, these are used as 1.) a switch, and 2.) a control over voltage, right? Or am I completely wrong on that? I'm looking for the easiest install and yours sounds like the more technical route. But if it is the right way to go, I will not skimp just because it is the harder way to go....I need to study up on relays a little bit, it sounds like. What I imagine is that when the door is opened, the door sill lights would come on. So using a relay would entail having a contact point on the door sill that is depressed to shut off the light, and pops out (when the door is open) to activate the light, correct?
Thank you all for your patience and responses. I ordered the parts yesterday, so I'm estimating they'll be here late next week. The project won't take up too much time, but I simply don't have a lot of time on my hands. I work 40+ hours a week and study about 30 - 40 hours a week. Needless to say, time is scarce. I estimate the project to be completed in 2 weeks, but as we all know, sh*t (oops, I mean LIFE) happens. I think I'll take photos of my progress throughout and post an update when it's all said and done with all the photos. I've had a few people say it's just a crappy idea, so I'd like to show them all wrong, haha.
The easiest way to add a light is just as you describe - tapping into an existing light and adding in a new light. That will work fine for most things. Sill lights probably don't use up a whole lot of power at all (They are LEDs, I imagine?). You can drop those in there and have no issues at all. In fact, a relay is probably a waste when we're talking current draws that small.
For other installations (many lights or higher-current lights, like puddle lights, aux headlights, amplifiers, etc), you have to consider how much current is being drawn and what it will do. If a circuit is meant to operate a tiny door light and you add a huge mess of stuff to it, you could blow a fuse, or worse: burn out a wire, connector, or even the control module. Even if it does work at first, you'll have to wonder if today is the day where it stops working.
For those applications, relays are the way to go. A relay is a mechanical switch controlled by an electrical signal. You still tap into the door lights (or whatever trigger you want), but instead of pulling the power to run the lights off that line, you just use it to operate the switch. Minimal current needed, so minimal impact on the circuit. The switch itself can switch whatever you want! You can run a wire directly to the battery and switch that, powering 200W aux driving lights, or your amplifier, or whatever else you want (make sure to use wiring and fuses that are appropriate for the application).
A third option is actually the one I'd recommend, but I don't know if it's an option in the subaru without looking in the fusebox. If there is a fuse specifically for the door lights, you can go to an auto parts store and get what's called an "Add A Fuse" kit and use that (Google for Add-A-Fuse to see what I mean). What they do is replace an existing fuse in your fusebox with 2 fuses - one is the original fuse, the other is a new fuse that comes with a wire you can attach to. That should give you the benefit of a relay without having to actually wire one up. Again, I don't know if the door lights are actually on their own fuse or not.