It's not rocket science, get one of them for example:
http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_204179_langId_-1_categoryId_165479Take off the trim with ash tray etc (just take ash trash out, remove blanking plugs from where switches are and so on, then you'll see screws)
Take the whole unit out, the disconnect wiring from your cigarette lighter - this will be three wires. (Thinner) One is for illumination - if your new one has no illumination ISOLATE the wire. Remove old socket, place new one there, maybe you need to glue it in (Araldite). Then all you need to do is connect ground and plus.
Yes spark plug is 20.8mm wide, so you need a 21mm deep socket, better than the T- handle thingy ...
Since you are gonna go somewhere and buy the 12v socket anyway get a few fuses as well (10A, 15A).