The following post will detail how to pick up +12v, switched 12v, dimmer and Ground inside the cabin of the car without hacking into wiring and compromising the OEM fusing system. This can be used for anything that requires 12v. Gauges, radar detectors, interior lights, ipod chargers, cell phone chargers, etc. etc. etc.
First thing, decide whether you need switched 12v or +12v (always on). Most applications will choose to go with switched 12v so your device will turn off when you turn the car off. Some people may choose the capability of operating their device with the car off, and still some may need a switch 12v and +12v source.
First off, disconnect the negative battery cable. You will save yourself headaches trying to find blown fuses after you short circuit something, because you will short circuit something.
To start, you need to remove the plastic panel beneath the driver side dashboard, and the plastic fusebox cover on the drivers side of the dashboard. use Gary's interior disassembly DIY's to do this.
( --steps 43 to 48).
Look up under the dashboard at the fuse panel. You will see several threaded connectors here. This is where you will get your 12v sources and ground. The stud marked 75x supplies SWITCHED 12v. The studs marked 30 (either one) will supply CONSTANT 12v. You can pick up a ground on the stud marked GND in the picture, that secures the fuse box to the bracket. Use appropriate sized crimp on ring terminals )found at any local autoparts/hardware store) to fit over the studs. PLEASE PLEASE PLEASE do not just dtrip the wire and stuff it under the nut, and then tighten the nut down. This is ghetto. Ghetto is bad. Ghetto and 12v can cause fires. Also, it is EXTREMELY recommended that you fuse your 12v source, whether it be switched or constant. You can do this with a simple in-line fuse holder, again available through any auto parts or hardware store. 10 amps fuses should be fine for any gauges, radar detectors, or chargers. For lighting just divide the TOTAL wattage by 12 and then move up to the next highest amp rating (ie total of 100 watts, 100/12 = ~8, so use a 10 amp fuse). The reason for fusing here, is because these feeds are protected by the metal fuses on the battery. Not only are these fuse more difficult to find then common AGC's or ATO's, but they have much higher ratings because the are main line fuses, which means that your device can possibly be damaged before these actual fuses blow. By using an appropriate sized dedicated fuse in your 12v line, you avoid this problem.
To tap into the dimmer circuit for gauges, etc. Remove the wiring harness from the back of the dimmer switch (you meay need to remove the headlight switch to get room to move around). The blue/gray wire is the one you want to tap into for the positive side of your lighting circuit (with most bulbs it doesn't matter which is positive and which is negative though). I personally hate the crimp-lock style connectors, and prefer a butt splice connector (one side of the dimmer circuit and the feed for you lighting go in one side, strip the other side of the dimmer circuit longer then normal, double it over, and stick it in the other side), but it's up to you what you use here. Again, hardware store, or autoparts store can help you get what you need. For the negative side of things, there is no need to tap into the brown wire. Ground is ground, and you can just use the same stud on the fuse panel, or forget about running another wire, and just tap into the ground you already ran close to the device (again, no twisted wires and electrical tape please).
To run wires/boost tubing to the engine compartment, you can use the existing grommet/hole above the accelerator pedal. If you have a drive by cable car (early 2.0s and a handful or VR's) this will be occupied by the accelerator pedal. Don't just knock out the grommet and lose it. Push it through from one side, and the slice a small cut in the middle of the rubber. Then run your wires/tubing trough the small slit, the rubber should provide a seal around your wire/tubing. The carefully push the grommet back into the hole (make take a couple of times before you can get it seated without pushing it through). Before feeding your wires through the grommet, wrap them in electrical/chafe tape for chafe protection. The blue/white wires and black tubing go through the grommet in this picture.
Modified by rs4-380 at 10:49 PM 6-20-2006