This is very easy to do. I did it to my '73 SuperBeetle. Find a Rabbit (Golf I) with a rear windshield wiper (if it doesn't have a rear wiper, this whole procedure is the same using the front washer parts but the mounting bracket will have to be cut from the car and welded to your car). Open the trunk and look to the right. There will be a plastic bottle with the pump in it, very old models have the pump seperate but it should be visible. 
Unplug the electrical connector and water tube from the pump. The pump is removable, but leave it in the bottle. Now, pull up on the bottle and remove it. There are sheet metal screws that hold the bracket to the body, remove them and pull the bracket out. Cut the electrical wire off as far away from the end as possible; you'll need a long wire when you hook it up. Salvage the water tube if you want to save a buck, but it is VERY cheap at the your local parts store. That's it for the Rabbit; take what you have and go to your Beetle or other model. Start by placing the bracket in the area where you want it to be mounted, it is a good idea to have the bottle in the bracket so you can see how it fits. Once you have it where you want it, drill some holes and mount the bracket. Slide the bottle down on the bracket. Moving on to the electrical side.... Plug the connector in; there are two wires, one should be brown and the other green with a red tracer. The wires are together as one; but, pull them apart and cut the brown wire and ground it by attaching it to a screw in the body. Take the other wire and route it into the passenger compartment to the wiper switch (you may need to attach another wire to it so it'll be long enough to reach the steering column). Once this is done, here comes the hardest part, find the correct terminal on the wiper switch to connect the green/red wire. If you have a '73 and later Beetle or Super, I know where it is :-), for other years/models you will have to get a test light or shop manual to find the correct terminal. '73 and later, find the large black connector from the wiper switch. 
Even though the wiper switch is on the right of the steering wheel, the plug is on the left side. Unplug the one to your left (it'll be on your right if you are lying under the steering wheel looking up at it). You will notice four female plugs on the top and one on the bottom. It is the one on the bottom/left that you need (I am working on getting a picture of this so you can better understand which plug is correct). You can get that very small plug in (you need to connect it to the end of the green wire that you ran from the pump through the trunk) from a stereo speciality store. It is called a DIN female connector. Get it, attach it to the end of the wire, push it into the socket and then push the large black connector back onto the wiper switch. Make sure that the DIN plug went in there good. Now, go back in the trunk and route a hose from the pump to the nozzle (the little black thing that squirts on the windshield) unhook the old hose and remove it. If you have a '73 and later SuperBeetle you will have to remove the black cover to do this; you will also have to drill a hole to access the hose to the nozzle. Fill the tank with water, turn the key on, pull the wiper switch towards you and it should squirt; if not, check the pump first (have someone pull the switch for you and listen to see if the pump is fuctional) if it works, check the hose. If it doesn't work, check to see if you have power to the pump when the wiper switch is pulled; if there is power, then you'll need new pump. If your nozzle isn't squirting the water on your windshield in the right spot, get a pin needle and stick it into the hole that the water comes out of and adjust it. Well, that is it. It took me longer to type this than it did for me to do this project on my car!
If you would like a more original looking install, the Mexican Beetle has had electric washers for years (Standard Beetle).Mexican Beetle bottle
The SuperBeetle electric washer bottle was only on European spec cars for just few years. European SuperBeetle bottle
As you can see here, the Rabbit / 924 switch looks exactly like the '72 - '79 Beetle and SuperBeetle wiper switch; only it has provisions for the intermittent function.
The intermittent wiper install will be added as soon as I finish this upgrade on my car and have all the information.
Send me e-mail for now and/or check out my letter that Hot VWs published.
