Speakers
The size of most speakers across the AU Falcon range are 5x7” oval speakers, which is a non-standard size. Note that 6x9” speakers can be used in both locations with some modifications, however additional holes will need to be drilled to allow for them, and special care must be taken for the front door speakers depth, as they can foul the window mechanism. Alternatively 6” round speakers can be fitted using off the shelf adapters from online marketplaces such as eBay
Popular replacements for these speakers are the Kenwood KFC-PS5796C, as they require no amp and are a drop-in replacement, barring the plug (explained in installation steps)
The only known exception to this is wagon body Falcons, which have 5.25” round speakers in the rear
The standard speakers used in the AU Falcon are known as a 250 Series Quick Connect 2-way connector. They are easy enough to identify as they have a T-shaped housing on them. There are many manufacturers for these connectors, and such it should be relatively trivial to find one from an Automotive Shop.
Note that the Loom end connector is the female terminal housing and that the factory speakers were fitted with male terminal housings.

Picture taken from Narva product page for a pack of both 1 male terminal housing (left) and one female terminal housing (right) (product code 56272BL)
The following miscellaneous information is known about the Speakers in the AU Falcon:
You will need a uncommon bit for the below listed jobs, an E6 E-Torx socket. These can be found in certain screwdriver or socket sets, but are a specialty bit in most cases. Alternatively there is known success using either a 5mm standard socket or a pair of vice grip pliers (however either of these options should only be used if a new screw is going to replace it, as it will likely damage the head of the screw)
Follow the steps below to replace the front door speakers:

Photo of screw locations
Remove the 4 screws holding the speaker into place and unplug the speaker from the wiring loom
After removal, it may be a good idea to clean up the door, as the surfaces where the speaker and door meet is prone to crust and rust, seemingly as a result of adhesives used at some point

Photos of the speaker screw locations in the door assembly, and the door post-removal respectively
If keeping the original loom, create or purchase an adapter with a male housing 2 way quick connector on one end, and the spade connectors for your speakers on the other
Manufacturers such as Aerpro can provide these adapters at time of writing, part number
APS58
If modifying the loom, cut and crimp the spade connectors for your speakers onto the loom. As much slack as possible should be secured to the door or the back of the speaker, to avoid interference with other components
Regardless of method used, note:

Example of a loom made for a set of Kenwood speakers

Photo of the wires from the back of the plug in the door side of the wiring loom. Note the wire with the Black line is Negative, and the one with the Green line is Positive
Connect your speaker to the wiring harness and reinstall into the door
Now is a good time to test that the speakers are connected correctly by turning on the radio

Photo of speaker installed. Note you can use the original screws, or the new screws and brackets from the new speakers if wanted/needed
Rear Sedan speakers can be upgraded to 6x9” speakers with relative simplicity. See the 6x9 upgrade guide for more information
Follow these steps below to replace the rear shelf speakers (Sedan only):

Photo of the screw locations for the rear shelf speaker. Note mesh covers not present (see note below)
The rear shelf speakers contain mesh covers similar to the factory speakers in the front doors, however these are prone to separation from the speaker assembly due to the direct sunlight exposure of these speakers. A common temporary fix is to carefully remove these covers to remove the vibration, however this leaves the cone more exposed to damage and debris.
Regardless of method used, note:

Example of a loom made for a set of Kenwood speakers
Connect your speaker(s) to the wiring loom and reinstall into the shelf
Now is a good time to test that the speakers are connected correctly by turning on the radio
Lack of images and information in this section due to project only involving Sedan
Ute rear speakers were optional, but should have the same 5x7” speakers as the Sedans. Ensure this is correct BEFORE performing these steps
Regardless of method used, note:

Example of a loom made for a set of Kenwood speakers
Connect your speaker(s) to the wiring harness and reinstall into the rear panel
Now is a good time to test that the speakers are connected correctly by turning on the radio
This section is not related to SPUD as it is a Sedan body type, but this has been created using an associated vehicle, Roadkill. Despite being a later model, these instructions should work without issue on AU Falcon Wagons
Follow these steps below to replace the rear C-Pillar speakers (Wagon only):
Undo the single 10mm bolt at the bottom of the rear seat bolster, and then lift the bolster to disengage it from a single clip in the top of the assembly
Theoretically the top clip could be disengaged without unbolting the bolster allowing you to push it away from the C-pillar enough for the rest of this procedure, but this is not recommended due to the risk of damage to the bolster plastic clip

Picture of the bottom bolster bolt and the clip and its original location in the top of the bolster respectively
Remove the speaker cover by gently unclipping it from the C-Pillar trim
This is simply done as a precaution against breaking, as the covers are thin and can become brittle over time
Remove the plastic C-Pillar upper trim. Note that this is held in with a screw in addition to clips along the C-Pillar itself
Without removing the bottom mount for the rear seatbelts, the C-Pillar trim cannot be removed completely. Simply slide it down the seatbelt out of the way for this procedure

Picture of the screw in the C-Pillar
1 plastic tab near the speaker itself

Picture of the plastic tab to the side of the speaker
1 plastic tab on the boot end of the panel

Picture of the plastic tab at the rear of the panel, in line with the interior edge of the tailgate
2 Torx T27 screws, holding in the Cargo Net clips. Once removed the clips themselves can be pulled out with moderate force
The removal of these is not strictly required if inconvenient, however this prevents damage to the panel on removal. If you choose to leave them, be careful of a small lip around the base of each clip

Picture of one of the cargo clips in the boot side panel
(If drivers side) 1 emergency jack & wheel tool compartment door
This is optional to remove, however removal of this prevents any possibility of damage to the plastic knob on the door itself

Picture of the emergency jack and wheel tool compartment
Remove the cloth panel itself from the inside of the boot
Remove the 4 screws holding in the speakers and remove

Picture of the screw locations for the rear speaker
Regardless of method used, note:

Example of a loom made for a set of Kenwood speakers
Connect your speaker(s) to the wiring harness and reinstall into the body
Now is a good time to test that the speakers are connected correctly by turning on the radio