Arrowed below is the ball joint for the steering arm.
It is worth noting that this part is the same part used for the hub ball joint lower down.
Steer the wheel towards you and you'll see it better. It's clamped in the hub carrier...
...and held in with 2 Torx 50 bolts and 16mm nuts.
Look behind the hub and remove the 2x bolts with an E18 socket. Slide the caliper off the disc.
Remove the Torx 30 screw from the brake disc...
...and remove the brake disc.
Use a deep 18mm socket to remove the nut holding the ball joint to the steering arm.
Hold the rear nuts with an 16mm ring spanner.
Now get a Torx 50 bit and hammer it into the head of these bolts. They will be rust and full of shit.
Do not be tempted to use a smaller Torx 45 as it will strip the head and then you'll
have to cut a slot into the head and remove it with a flat blade screw driver.
The steering arm will now pull down with the ball joint attached.
It gives you better access for the ball joint remover.
Slide the remover into place and tighten down the bolt to make it push the ball joint out of the steering arm.
It will suddenly bang and come free, usually straight into your face.
You can see on this particular one that the rubber was damaged.
Here's the lovely new shiny one.
Push it into position and line up the holes with a screwdriver.
Push the bolts through, place the nuts on the back and tighten them up to 40Nm.
Put the steering arm over the end of the ball joint and fit the new nut.
Depending on the brand you refit, you may have to hold the end of the ball joint to stop it spinning
as you tighten the main nut up. The OEM smart ball joints don't spin.
If you can, torque this nut up to 46Nm. Refit the brake disc, caliper and wheel and you are done.
Ball Joint Separator
Most separator tools are listed as 20-50mm which is a 20mm jaw and 50mm opening. These will be fine.