The most important thing to remember when installing electrical devices is "current draw". When adding devices to an existing circuit it is important to determine that everything between the power source and the device being added is sized appropriately.
When designing a new circuit you first add the current draw of each individual device to determine the total load in amps and then you size the wire accordingly and use the appropriate fuse to protect that size conductor.
The reason for the use of relays in a circuit can most easily be explained by considering the most basic engine "starter" circuit. If it wasn't for the use of a solenoid (large relay) we would have to install a huge ignition switch in the dash large enough to handle the total load of the starter motor and run "jumper cable" size wires to and from it.The use of a relay allows us to reduce the size of the conductors running from the switch to the relay because the only current running through the switch is the load of the relay not that of the devices it controls. And with the advent of ECU/ECM's the use of relays was imperative otherwise the ECU would have been as big as a lunch box and extremely expensive.
My best advice to someone who is considering adding an additional load to an existing circuit would be to measure the maximum existing load with an amprobe there by removing any guess work. The one I have in my tool box is a FLUKE T5-600 and they can be found on ebay for a little over $100. And the best part about this particular meter is you don't have to disconnect any wires to use it.
I know I didn't directly answer your question @Cameron Roberts but I hope this helps someone trying to tackle some basic wiring mods on their own.
Posts by Slunglow
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Thanks @Painter for the videos all very good info. And thanks also for the PM but the dark side forum Nazis got to it before I did.
But it's all good now I found the light 
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This is off subject ... But I love the multiple quote format on this new kickass sight. Uh oh I said ass.
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Lol I kinda thought you might like it Stylin', it's definitely different. It's the future wife's I bought it for her for Christmas a couple years ago. It was a floor display and I bought it on Christmas eve, whew that was close.
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I'm looking forward to your engine build thread Rab. I guess since my Hahn turbo is still sitting on the bar in the great room (not very fast there) I'll settle for Post # 16
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If anyone is having trouble registering they can contact Rabtech like I did and he will get it done painlessly.
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Did you bring your macho, manly & totally badass tool box with you?!
Yup gotta have my tools to setup the new Still
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Damn it man, you guys have been busy this weekend. I got a lot of catchin' up to do. I jumped out of the boat and was treading water for a while but Rab threw me a line. Thanks Rab and others for doing what so badly needed to be done. Now who brought the beer ?
