Mk2 Unos, I believe, have the wiring provided. Look for a plug with two terminals - black (earth) and some other colour, probably brown, with 12V.
On the Mk1, brown is the lead for the radio power. It may be up to the job, because on Unos with the lighter socket, the lighter does indeed run off the same supply as the radio. If the Uno has a radio fitted, try pulling the fuse with the 'cigarette' icon above it. Does the radio stop? If so, you've found the answer (use the radio power lead).
Otherwise, if you find the radio is controlled by the 'Services' fuse, then you will have to poke around behind the fusebox or look in the dashboard for the intended wire. Be careful - the labels are ABOVE the fuses, not below. You might like to check the interior light - that should be on the Services fuse, but for some Unos I've had, it was on the Lighter fuse.
Really, you can't say for sure what fuse or wire colour you may encounter, because Mario may have made up the wiring loom quite differently or Luigi may have plugged stuff into the 'branching box' randomly in order to make it work. Have you ever looked at electric window wiring? I'm sure it's made up on the day. And the interior light has NERO, ROSSO, BIANCO written on it (black, red, white). The three wires are coloured black, white, and white/black. That proves that you can't possibly tell what colour the wires are going to be, because it depends on what colours were available that day (of course, I don't actually know this for certain either: it may instead be related to the colours of the team playing against Torino that night).
If you plan to attach a heater or inverter to the lighter socket, I think it would be better to use a 10 or 12 gauge power cable straight from the battery, with an in-line fuse holder near the battery (you can find this equipment where you would find car audio amplifier install kits).
The fascia for the heater controls has an illumination bulb (Mk1, at least). Wire colour for dash illumination is yellow with red stripe - at least, provided the month had an R in it.
Black wires are always earth (except, if they're brown like someone else said). Around the back of the glovebox you will find an earthing point, floating in mid-air of course. Make up an earth wire of medium thickness and attach to the spade terminal point, or put a ring terminal on and fasten under one of the nuts that holds the heater onto the bulkhead (front of the cabin, under carpet). The bulkhead is always earth, even in leap years, because it's part of the chassis.
Of all things, you can make the hole for the lighter socket using a pair of scissors... Go carefully and you'll be able to carefully increase the hole size as required.
Also I believe that there are two sizes of lighter sockets in the world - European? (large) and Japanese - and others? - (small). If you put in a European socket, you may have problems like I do with the iPod charger/Nokia charger, etc. falling out of the FIAT/Lancia/Alfa/BMW sockets. In the case of the iPod, this cuts the line-out off, and pauses the iPod (it's a pain). I'm considering replacing the standard lighter socket with a Japanese one to fix this problem... particularly annoying in the X1/9 although at least I can use my knee to hold the plug in...
If you've read this far, thank you, congratulations, hope you enjoyed the festivities, and Happy New Year... I hope you have a prosperous one, maybe even one where you have enough time to fix a FIAT or two...
Remember, Rome didn't rust in a day.