I do this 'quick fix' on all new cars I buy, as the paper cone speakers supplied by most manufacturers are pretty dire. The aim is to provide greater clarity and less distortion, not to produce a mobile discotheque - gigantic subwoofers are not my cup of tea and I take great care to ensure the mods are as invisible as possible.
Step 1: remove the door card.
On the Custom, the door cards are held really tightly by a dozen or so hefty clips and a total of 5 screws, located behind the door pull, around the side, in the centre of the bottom bin and two right underneath the card. Removal is straightforward using a T20 torx driver. Using a trim removal tool or nylon spatula to pry between the card and the door, pull very firmly on the card. The effort required on the Custom is greater than I have ever come across before, but it is possible.
There is a really annoying final screw located half way along the window at the top. This can only be accessed once the card has been pulled half away and is a right bugger to shift!
Step 2 (optional): add sound deadening.
You don't have to do this, of course, but I have access to cheap Dynamat Xtreme and as I'm 'in there anyway', I usually do it. If you haven't used this stuff before, it's basically like very thick sticky-back plastic, and the great thing is that you don't have to stick it in one piece. Just cut the bits you need and slap it on, taking care to roll it as firmly as you can into all the nooks and crannies with a seam roller. It is advisable to do the inside of the door as well, especially directly behind the speaker. In addition to providing better quality sound, it also makes the doors close with a pleasing dull thud, rather than a metallic clang.
Step 3: Fit the new speakers.
You need some 165mm diameter speakers (I always go with Focals) and - crucially - some adapter rings. The hole left in the speaker support panel is about 20cm in diameter, meaning the 165mm speakers can't be fitted directly. I tried a local car accessory store and Ford themselves, but no one could find the right reference. I therefore took a chance and ordered some Ford Focus adapters at a cost of €15; fortunately they are 100% compatible with the Custom and screw straight in.
You will also need to drill a small hole in the speaker mount on the door to feed the wires through (see photo). Then just strip the wires, tin, splice together and finish with some heat-shrink tubing.
Once the door cards have been put back, you can sit back and enjoy. The difference in quality with this relatively simple setup is very noticeable: greater clarity, crisper and you can push the volume much higher than with the standard Chinese efforts without any distortion.
Photos:
Door card removed, outer door skin Dynamatted:
The crappy Chinese speaker Ford gives us:
Note the difference in diameter between the OE speaker (left) and a standard 165mm speaker (right)
...which is why you'll need a pair of these Ford Focus adapter rings (at the time of writing, no one had the part reference for the Transit Custom):
Slap some Dynamat Xtreme on the inner door skin:
Drill a small hole in the speaker mount and thread the wire through: