The domelight wire should be a pink wire on the back of the fuse box in an 8 pin plug. Not 100% so make sure you check the wire before just connecting.
Also, you don't want to power it with 12 volt power. 12 volts can actually burn up an LED. If you have LEDs hooked in series it should be fine, because less voltage is going to the LEDs. You can also use a resistor inline if you are just powering one LED.
Here are some good sites about LEDs, especially if you are making your own groups of LEDs and not just buying a pre-built light bar of them. The pre built ones from stores should be fine to hook up, they come with resistors and crap in them I believe.
http://www.theledlight.com/ledcircuits.html
http://metku.net/index.html?sect=vie...calc/index_eng