I just carried out this swap so I could have an OEM looking headlight switch on my bike. I wasn't planning on doing a guide because I already had modified wiring on the bike, but after completing the mod I realise there are still some pointers I could post up. Sorry that this isn't as complete as it could be by any means, but anyone serious enough to do this will definitely find this useful and enough to do the job.
I don't know the specifics of which 955i models came with the headlight switch, I just did an eBay search and one of them had it. Here is what mine looked like before I put it on for the last time:
The main plug on the 955i harness will connect directly to the 675 loom, the clutch switch will not. The 955i clutch switch uses a flat 3 pin plug identical to the 675 indicator relay plug. The 675 clutch switch is a 2 wire plug that has to be cut off and connected to the 955i harness. One is earth, the other is a black with yellow stripe for signal. I forget what the third wire on the 955i clutch switch went to but it can be sliced off without issue.
While the main plug will go straight onto the 675 harness, there is obviously not facility for the switch to straight up work for the headlights. Every other electrical switch component on the switch block works directly after plugging the 955i switch block in. The switch block itself bolts straight onto the clip on too. If you find the plastic locating dowel has been snapped off like mine was, the location in the switch block can be drilled and a grub screw would in to operate as a dowel without issue.
Triumph was lazy doing the wiring for the 675. Heaps of plugs and wires remained in the harness despite the bike not having a headlight switch. In this case, the pin slots on the bike side of the plug to add the headlight functionality are still there, simply empty. The connectors use these pins:
If you want to do it properly like factory, you'll want to get a few female pins and have the ability to crimp and slot them into the factory plug. Not complicated but if that's not your style, simple cut and solder hackery will do the job...
This crappy pic I put together shows the before and after for the wiring. Initially, the starter/headlight relay splits off and goes to both the front clip and the switch block (IIRC it still has to go to the switch block for the OEM headlight switch). You need to cut the low beam wire where it connects to the junction headed to the relay, and run a new wire from the switch block plug pin 6 (red w/ yellow dots) to connect to where the blue wire was cut. If you're particular like me, you can also connect the running lights to the switch.
This is the 955i main plug. Bottom left, left to right is the low beam output, high beam output and clutch switch signal on the 955i plug, none of which have to be moved.
If you have a projector lens HID conversion like I do, you'll have to carry out a modification to the switch. If not modified, switching to high beam with the 955i switch block alternates between low/high beam. For a twin projector set up the low beam HID globes stay illuminated, with the high beam switch activating a small coil in the lens that opens up the projection pattern to widen the light distribution. Obviously if this is combined with the 955i switch you will have no lighting at all when switching to high beam. I pulled the switch apart and crimped two wires internally on the inside of the high/low switch, leading to the low beam always having power on that circuit and the high beam also gaining it when the switch is hit. As the low beam is switched on/off at the headlight switch (as opposed to the high/low switch), all works well. No pics of this sorry.
Attached is two wiring guides and jimmytb's set up of the completed job, much appreciation to him for one of the wiring pictures attached as it was very useful to me.