Can't remember who asked about this a while back, but as I had the bike in pieces yesterday I took some pictures of how I've wired in my accessories properly rather than just piggybacking onto existing circuits. 

You'll need:

  • A 4- or 5-pin relay - mine was from Halfords, make sure it's 12V and can handle whatever current you're planning on putting through it
  • A handful of inline fuses
  • A selection of crimp spade connectors
  • Plenty of heat shrink of various sizes
  • Obviously cable ties
  • Wire capable of carrying the current you're after

A relay is just a switch, operated by a magnetic coil instead of a button. The coil takes an absolutely minimal amount of current (milliamps) so can be safely powered off pretty much any existing electrical circuit that's on the switched live. When the coil receives power it connects the two other terminals on the relay, providing your accessories with their own direct feed from the battery, rather than drawing large currents through circuits which were never designed for it. I was fitting the following:

  • Oxford heated grips
  • TomTom Rider sat nav cradle
  • USB phone charger

All of these things say they can be directly wired to the battery, but that's a bit crap. The grips say that they have some automated system that can tell when the engine's not running and you're draining the battery and shut off automatically, but I don't really want to be relying on things like that. Again the sat nav cradle - when there's no sat nav on it (which is most of the time) the power pins are exposed. You really don't want those live when your bike's sat out in the rain for hours. Same goes for the phone charger - it's just a power cable hanging off the side of the bike for anyone to fiddle with, don't need that shorting or getting wet and having a flat battery. So proper switched live it is. 

In-line fuse from the battery to the relay feed terminal. All the devices tell you what current they need, so pick the smallest fuse you can get away with, but don't cut it fine. Things like heaters are inductive loads (they draw more power for a fraction of a second when you first turn them on), so them might draw 5A when running, but 8A on startup - you need to account for that when picking a fuse. 

This wire goes to the feed on the relay, which on mine was the top terminal. 

The load terminal from the relay then gets split however you like (I used a load of ring crimp connectors bolted together with an M4 bolt and a nyloc, then heat-shrunk), then via another, smaller and appropriate,in-line fuse to each individual device. The phone charger has a tiny 1A or 0.5A fuse, the grips have a 5A or 10A I believe. The device should tell you what to use. 

I labelled all the fuses so I can easily change them should one go pop. 

All you then need is a switched live to the relay coil. Now technically you can just cut into any circuit on your bike which is switched, like the headlights. The relay coil only takes a few milliamps so you're not going to cause any problems. However firstly make sure you know what you're cutting into, and make sure you solder the connection properly - if it comes loose (as those in-line piggyback connectors do) you'll lose the electrics to all your accessories. 

After some Googling I found the fusebox in my bike has a big busbar which is the main switched live, which then feeds the individual relays. It was pretty easy to unclip this from the fusebox, solder on my relay feed, then via another in-line fuse (0.5A) feed the relay coil. This way nothing has load it's not expecting, there are no wires to come loose, and it's all beautifully neat. No photos of that though I'm afraid, not pulling that all out again. 

So about five fuses in all, but it means that:

  • If the grips go wrong, they should pop their own fuse, leaving everything else in place
  • Same for the phone
  • Same for the sat nav
  • If the load side of the relay goes wrong and draws too much current, there's a fuse from the battery which'll go to stop the battery from draining
  • If the coil of the relay goes wrong, the 0.5A fuse will pop to avoid excessive current being drawn through the bike's switched live circuits

Between all those, that should be all aspects covered. Been running the setup for a few months now, no problems whatsoever. 

The phone charger is an Ultimate Addons phone charger kit, and it's brilliant. It has a little transformer box (with a switch that we don't need in this case, just make sure it's on not off, duh) and plenty of wire. It comes with connections for straight-to-battery but cut those off and wired it in. 

Very neatly terminates in a waterproof WeatherPack connector which I have velcro'd to the side of the bike. When I want it, it just comes up over the fuel tank and into my tank bag. You can then buy different tails for different devices (iPhone, MicroUSB, MiniUSB etc) so you don't have to change it every time you get a new phone. Means I can have my phone in front of me, fully charged at all times on long journeys. Same goes for charging your iPod if that's your thing. 

Heated grips are Oxfords, and they're brilliant. I looked at getting some aftermarket hand guards for the winter but alas I can't get them for the Fazer, so these and some decent gloves will have to do.

Overall view of everything once in:

Job's a good'un.