Just got my H7C bulbs in, nice job to do when it's -3.
I found it was much easier to install the ballasts by swinging the coolant reservoir out out the way by undoing the 3 screws holding it in place (no need to undo any hoses). The otherside, just remove the plastic cowling that runs down the side of the engine bay.
Having these bits out the way, you can then easily strap the ballasts to the metal brackets inside the wings, helps keep things tidy.
If you've never removed/installed dipped beam bulbs in a 156 then either read up or get yourself a make up mirror from Boots for a couple of quid. Twist the cover of the back of the light enclosure and with the mirror have a look at how the clips hold the bulb in. I find it easiest to first pull the electrical connector off and move it out of the way before undoing the clips otherwise they tangle up. You push the spring clips (at the top of bulb not bottom) towards the front bumper and while pushing in, push sideways.
There's a small tab on the metal base of the bulbs, this goes to the top of the fitting, just in case you forgot to check when you pulled them out.
Check which of the electrical connectors is the power, I found driver's side had a black and a brown cable, black was live. Passenger side has a yellow and a brown cable, yellow was power on this side.
Next do a test of your bulbs out of situ, may aswell check it all works before clipping it all into position. (I actually did this before fixing the ballasts) The instructions told me to put the ignition on (no engine) and put the lights on for 10 minutes, this does something good to the ballasts apparently. It also runs the battery down. After 10 mins I started the engine and turned the lights on and of a few times to make sure the bulbs were starting up consistently.
So you have your ballast strapped in position, you've cut a hole in the rear cap for your cables and grommet to thru. I found the less faffy way was to leave the rear cap on my workbench and put the HID bulbs in position first, put the clips on then get your mirror out and check the bulb is seated properly as it has the tendency to move slightly out of position when you hit the wires with your hands. Now your bulb isn't going to move when your fiddling with all the connectors .
Connect the + and - cables up then get your rear cap and pull all the wires from the bulb thru until the grommet slots into the hole. Fix the cap onto the lamp housing then make all the connections.
That's how i did it anyway and i'm bored so I thought I'd write about it.
I now have some H7R (longer length) bulbs spare, if the GT wasn't covered in snow I'd check which length bulbs that has, maybe the longer ones will fit and I'll have spare set.
Fitted some clear side repeaters yesterday, amazing difference, kind of brings the car together.