Stay away from using the lighter as a tap-in point. Everyone does it but a) it's a pain to access, and b) it's a lightweight connection and repeated tap-ins can overload it and the attendant wiring and c) cigarette lighter plugs are crappy devices and often short circuit - even if you don't use one, say, as a non-smoker, if the plug is in place, some wally will depress it one day and if it is dodgy it can short. I keep mine out altogther.
Set up a new insulated and fused multiple point power-board block under the bonnet or on the interior firewall (much less desirable due to restricted access and considerations of bringing heavy current cabling inside cabin), that you can then use for multiple hot points. Pull power off the main fusebox next to the battery. A test light will define the take-off point for you that is only on when the ignition is on. If in doubt about your skills get an auto-sparky to set it up.
Make a grommeted access port through the firewall to bring power inside the cabin. Take precautions against chafe and fuse everything - not to protect your new component but to stop a short from frying the entire wiring system to which it is connected. Best side to do this is the LHS, in front of the glovebox, approx 2" (50mm) to LHS of the two heater pipes that pass through the firewall which you can see from both inside the cabin behind the glovebox, and from under the bonnet behind the battery box, and use as a reference point. Drill at that 2" distance from and at the level of the half way point between the two pipes. There is some latitude to this but not a lot, say +/- 1cm for comfortable access. The interior fibreglass and rubber-backed insulation is thick there and will wind up around your drill bit - it's too thick to use a chassis punch on. The rubber is sticky stuff when it gets warm on the bit. Start with a small 1/8" hole and progressively enlarge it to about 5/8" or 7/8" for which you can get standard grommets. It helps to remove the glove box to enable access and identify your drill site, and remove the battery so you can hold a (compact) drill straight.
Hope this helps.
CP_e Standback & PNP; CP_e 3" SS Downpipe; Corksport FMIC with Top-mount K&N filter & OEM Ram CAI; Turbosmart BOV; Dashhawk; Prosport Boost Guage; JBR solid shift bushes; DBA 4000 Wiper-Slot front rotors; Hawk Ferro-Carbon HPS Street front brake pads (@ 69,000km); Sumitomo HTRZIII's in 225/45 x 18