I've just installed an active Focal BUS 20 sub in my car.
Every time I switch the car to the accessory position I get a noticeable pop from the sub.

I'm using the stock HU and tapped into the pre-out cables going to the bose amp in the boot. I'm then powering it from the pink/black cable going into the bose amp.

The gounding point seems fine. What else could I check to try and get rid of the pop?

 

Tell us more about your wiring then may be we can help further.

There usually +12V constant, +12V ACC (called remote on amp) and GND connections on the amp for power, make sure you don't connect your amp's +12V constant to car's +12V ACC, means the amp gets it's power only when you turn the key, the power surge could create pops when start.

Some better amp circuitry has "soft start" built-in, like the VDO (Japanese made) I have in my daily, it eliminates all sort of start up noise.

 

I read your post second time, now I'm pretty sure the problem lies with your wiring, you must have tapped all +12V constant and +12V ACC (remote) on the amp to ONE +12V going to the Bose amp? If yes, you will have to separate the +12V constant and +12V ACC by running a wire either directly from the battery through firewall (make sure you add in-line fuse to protect your amp) to your amp for +12V constant, OR tap the power from the cabin fuse box, use one of these



to connect to "Audio System (11)" in the picture below for +12V constant.


For the +12V ACC, you can just leave it connected to the power you already tapped off the Bose amp OR you could tap off the (2) fuse in the pic above, this should eliminate the pop.

BUT please confirm your existing wiring first!

Hope this helps.