ezdan
Internal wiring can look extremely difficult but it is actually pretty easy if you are patient. As far as the hole size in the bars that depends on the gauge wiring that you are using and what switch housings. First you have to determine the tools you need.
I use: Soldering Iron, Solder, Heat Shrink, Electrical Tape, Hair Dryer, small length of nylon string, Long piece of bendable wire, and Electrical Pin removal tool part number 381-087, wiring diagram of the bike, or digital camera to take pictures of Stock wiring.
1. You need to determine the length and gauge wire that your motorcycle uses. This is based on the motorcycle and switch housings, and factory wire size.
2. It’s a lot simpler to do everything off the bike until you are connecting the wires to the bikes main harness. .
3. Once you get all the wires cut to length you need to get them through the bar. The easiest way I have found is if you get a small piece of nylon string, tape off all the holes that you do not want the wire to go through starting with left side and working to your right. Once you tape off the holes in the right side of the bar. Start feeding the nylon string through the hole once its started you want to use the hair dryer to blow the string all the way through the bar and it will come out the bottom of the bars. (This will be a little harder on T-Bars and you will have to keep working the string in and out of the bar until it comes out the correct hole, Patience is the key).
Once you have the string out both holes, tape your wires to the string using the electrical tape (stagger the wires so that you do not have a large bundle coming through at one time). Do not use the string to pull the wires through only use it as a guide for the wires. Push the wires through the bars and gently pull on the nylon string. .
Another Method is to use a piece of bendable wire to navigate through the holes and tape the wires to the bendable wire and pull the wires back through the bars. (Old Broken Clutch cable or Throttle Cable works well for this)
4.Once you have all the wiring through the bike use the Pin removal tool and remove the stock wiring out of the connect or cut and re solder the wires. I prefer removing the wiring from the connector to prevent damage to the wiring or the connect while soldering. Important (Extend one Wire at a time)
5. Make sure you use great quality solder and heat shrink to prevent any damage to the motorcycle or wiring harness.
Good Luck and Hope I answered your questions.