Required:

iPod Classic (6th Gen+, let me know if it work with earlier models)(Thick or thin should be fine, I've only done thin)

3.5mm to Bluetooth Transmitter

iFlash SD Card Board (Single or Quad, the Dual will not work)

Soldering Iron with fine tip

Solder + Flux

Thin Gauge Wire

Drill with 1mm drill bit (And others)

Thin round file

Hot Glue Gun

Something to use as a button

A Steady Hand + Patience

Rockbox

Step 1: Prepare the Bluetooth Board

I ordered 3 of the bluetooth modules and each one contained a different board (audio quality, battery consumption and remembering last paired device varies between the 3) , so it's a bit of a lottery. The best I found was the one in a white case with board TS-BT35F01, if you search for these they should still be available. Obviously it has to have the form factor of the one in the picture for button/LED placement.

Prep the board by removing all the cables, and carefully removing the charge port from the board (I used a pair of wire cutters without issue).

Step 2: Prepare the IPod Frame LED

Remove the logic board from the case just leaving the front panel, screen and the aluminium frame and drill a hole using the 1mm bit on the right hand side a little above the centre (Where I put the green 'x'). I found that directly above the LED was too bright, and 10mm in from the side was too dim so measure twice cut once (12-13mm is probably a good distance).

In the second picture you can see (Very poorly) the notification LED. Once the hole was drilled I held the glue gun against the hole and put a little glue in, then pushed it flush with the front of the unit with a sim card tool.

Step 3: Prepare IPod Frame Button Gap

When you place the bluetooth board in the iPod you'll notice that the button doesn't quite sit flush. Mark with a pen where the center of the button lies and file it down around 2/3rds of the way through the side (Better too much than too little here).

Best to remove the frame for this and file it separately to avoid the shavings/dust short circuiting stuff on the iPod logic board. However if you are lazy like me just masking tape over it and use a paint brush to sweep any excess after.

Step 4: Power Wiring!

Ok now for the tough part, using my super helpful wiring diagram you can see that of the 5 pins in the battery connector they are VCC VCC TEMP GND GND (Pull the plastic connector off to see all 5). You'll want to remove the board to get the best access, get a decent amount of flux and carefully tin the outside pins for VCC and GND. Be gentle with them because they are a pain to put back on if they come out (But if they do you always have a spare, right?).

A smarter man might solder directly to + and - on the battery terminals, I tried this and it blew up. So back to the pauper method for me.

Firstly I measured the length of the wires and soldered them to the BT board, it can get very fiddly and tricky to solder stuff in the confined spaces in the iPod. Once they're soldered to the BT board place the board in the iPod and solder the connectors to the iPod battery terminals (Tin the terminals first, seriously).

Once the wires are soldered on, they should look a little like the second picture, try and solder as close to the base as possible as putting the plastic battery connector back on top can break the solder if too much pressure is applied.

Now's a good time to plug the battery in and check that the bluetooth board is getting power.

Step 5: Audio Wiring!

The part gets a bit tricky again. You'll need to remove the headphone/hold switch assembly from the back frame. Once you've done so remove the plastic shell from the underside of the headphone jack to gain access to the solder pads. On older iPods (.. all of them?) the plastic can have degraded and need to be picked off in parts. Either way remove enough of it to be able to solder to the headphone jack pads.

Using more of my super helpful wiring diagrams note the 3 solder pads on the left hand side are standard TRS, Ground-Right-Left from top to bottom. Using my powers of deduction I reasoned that these should be connected to the G R and L connectors on the bluetooth board.

At the end of this step it's a good idea to test everything again to make sure no solder points have broken/nothings short circuited. But you should have a working prototype!

Step 6: Finishing Prep

Using the same wire cutters from earlier snip a bit off the side the the iFlash connector, this is because the button gets in the way of it otherwise (See Image #2). I have done this twice with iFlash Single boards, triple check before you start cutting bits from your iFlash Quad but it should be fine too.

Before you put the iFlash connector in you may have noticed in most of the pictures there is one of the foam cutouts from the iFlash board behind the bluetooth chip, put that in there to stop the chip being pushed backwards by the button press.

Also from Image #2 mark on the case where you need to drill the whole for for the button in the backplate, it will need to be quite low on the case angled slightly downwards, I've not got it to work directly on the side. For buttons I have tried;

Taking the tips from breadboard buttons
Trimming down the button from the original bluetooth module
A tiny screw

None of them work amazingly, although the screw was pretty good size-wise.

Once you find something to use as a button place it in the hole you drilled in the backplate and use hot glue to secure it from the outside (will make your life 10x easier). Once you've double checked everything's still working you are ready for the last step..

Step 7: Placing the Unit Back Together

The last step can be fiddly, very carefully, starting by closing the right hand side of the case (the side without the button) and applying pressure to close the left hand side evenly, if there is a lot of resistance try and pin point where it is. You maybe need to remove some of the plastic from the switch on the inside edge and apply spots of hot gun glue to hold everything in place while the iPod is closed as it's pretty tight in there now.

Once it's done you should have something similar to the video! Congrats!

Step 8: Notes and Thoughts

So while having an iPod classic with integrated bluetooth is pretty sweet there are a few drawbacks:

The range sucks. Seriously sucks. As long as you have line of sights you're ok but I cannot have my iPod in my back pocket as my body will block the signal. You can alleviate this a little by replacing the front plate with a transparent plastic one. Or if you are an aerial wizard replacing the aerial on the board with an external one.
Battery life is not as good. If I don't use the bluetooth and stay cabled I get 24-30h playback time, using bluetooth get's me 10-12h. But that's still pretty good.
Having to carry a 30pin charger.

Pros:

Cheaper than an iPhone 7, and you get to keep the audio jack.
You can set the volume from the iPod, other mods use 30 pin BT connectors which use lineout volume.
You can switch between headphone jack and bluetooth pretty seamlessly
Having a 512GB storage, bluetooth and all day battery life is pretty neat.

Feedback and/or comments on my awful soldering always appreciated.