Most of all the new head units (radio, stereo, wireless depending on your age group!) now come with a standardised ISO socket on them to facilitate ease of fitment. Likewise most new cars come with a standardised ISO plug so new head units can be swapped with relative ease.
VW started using this system on passenger cars in the late eighties.

Only cheap head units don't come with the ISO socket, and to me they aren't worth fitting!
The preferable way to fit a new head unit to your vehicle is to fit a ISO plug to your existing wiring, this saves you chopping the wiring from your new head unit and rendering it useless if such a time arises that you would like to fit it to something else, you also have the benefit if you fit a ISO plug to your vehicles wiring loom that future head unit replacements will become so much easier, and the added bonus if your vehicle is left parked up for long periods it can safely be removed and stored so it doesn't get stolen.
ISO plugs or sockets can be purchased from good auto accessory shops for around 10 GBP2004 each.
Attaching the plug to the wiring is not a difficult task, soldering being the preferred method using heat shrink tubing to cover the soldered joints to prevent shorting to ground or other connections. Do not in any circumstances use "chocolate block" connectors or "scotch lock" connectors as audio quality will be compromised and there is the chance of electrical short circuits as there are exposed metal parts.
Chances are that if you are fitting a Head unit you are replacing the standard radio, which only has one speaker so if you don't have any other speakers now is the time to fit them and run new wires to the dashboard.
The standard wiring behind the dash is adequate but if you have a split charge system and a second battery then it seems stupid to use any of the standard wiring except for the earth, running a new power wire from the second battery to the dash is a must, fitting the appropriate fuse in line. Make sure if you take this option to insulate the original power wire (Red) with insulating tape and cable tie it out of the way so it has no chance of shorting against any bodywork, do not completely remove the wire as you may want to reuse the wire in the future if such a time comes that you sell the vehicle and want to restore the original head unit back into the dash, or want to use the feed for another application.

If you are using your new head unit in a camper then chances are you are going to want to listen to your music while you are camped up and having the ignition on isn't a good idea, the solution to this problem is to use one power wire from your battery and on the head unit connect the red and yellow wires together, this is quite safe and most decent car audio manufacturers give you this option on the wiring by making the radio loom with several connectors which allow you to join the red and yellow together. There will be a provision to completely turn your head unit off so it will not draw any power when turned off.
Most new head units are now "face off" which means the front part of the radio is removable to try to help combat car crime, so the head unit will be turned off when the face is removed anyway.

The mounting of speakers isn't just as simple as bolting them down, special consideration has to be used to the positioning to make full use of the stereo effect. Also in larger vehicles if the speakers are put right to the back of the vehicle the music will sound odd as the speed of sound is relatively slow compared to the speed of light which electricity travels at. If you have speakers fairly close to your head and some miles away then sound will take longer to travel from far away to your ears and will sound "out of phase" and flat, a way to combat this is to try to design your audio system so you are equidistant from all your speakers, within reason of course.
From a standard power head unit (I.E., without any amplifiers) then the rear speakers don't want to be much further away than in the facia under the rear seat. An ideal place for front speakers is in the bottom of the front doors, angled upwards if possible. Please remember that VW window seals are not designed to keep water out of the cab and the plastic membrane behind the door trim is your primary seal, if you cut or remove this membrane then your speakers will get wet, not good if you use paper cones!, and if your speaker doesn't get wet your door trim will and it will buckle as it dries out. If you do fit speakers to the front doors use some fresh plastic sheet fitted fairly slack so you create a indent for the speaker to sit into, also make sure it doesn't vibrate!
The use of "tweeters" is an inexpensive fix for badly positioned speaker giving you stereo effect right where you want it, in the cab. Treble is directional, bass isn't, that's why you can hide a sub woofer in the boot in a car because you feel bass. Tweeters can just be tagged onto some existing speakers without any ill effects to the head unit or the sound quality of the speakers they are connected to.
Unused speaker connections should be insulated as not to short to earth or each other, if the wires are allowed to touch this could cause damage to the internal amplifier of the head unit.
Likewise speaker wires should never be joined together or put to earth or you are going to have problems.

Pre-out connections are for the use of and external amplifier, they look like phono plugs on the back of the head unit, these are connected via a RCA cable to the amplifier. Amplifiers need their own power supply. Amplifier fitting kits are commercially available and if its your first time fitting and amplifier then this is probably your best option as all the correct rating power cables are included along with good quality RCA cables, fuses and depending on the kit some speaker cable. The easy way to remember which is right or left , Red = Right ( the 2 R's) with the other being white, which is obviously left.

Speaker cable, bit controversial this one, some say the more you spend on cable the better the sound, its down to personal preference and the size of your wallet, there's no point spending a fortune on cable to some crappy paper dual cone speakers and a crap head unit, likewise you don't want to run cable from your amp to a high power sub that is similar to bell wire! Horses for courses and all that! Oxygen free is generally the high end of speaker cable and to me is worth buying, its called oxygen free cable due to the way it is extruded not by the air trapped between the wire and the insulation as some people think! "what happens when you strip the insulation back? doesn't the oxygen get in?" oh how we laughed at the audio board of the local Halfords at the "rude boy" with his Toyota something or other...

ISO Connections.

Below is a ISO plug, this is the part that is attached to the wiring loom of the head unit. Note how they cannot be connected into the wrong sockets by the location of the lugs, one is towards the top, one is in the centre.

	Connector A.	Power connector.
		Please note some terminals on connector A are left spare.
	Connector B.	Speaker connector.

Other wires you may come across that aren't listed.

Brown - Telephone mute. If you have a mobile phone fitted to your vehicle then when the phone rings or is in use earthing this wire will mute the sound from the head unit for the duration of the call, once the call has finished sound will be restored.

Pink / Black - External amplifier control, used on amplifiers that have a EXT.AMP.CONT terminal only.

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