Hi wheelybins,
Welcome to the forum!
I was putting off the job of replacing the rear speakers for a while also (about 4 years!)
But, once you get started its not too bad at all!
First off, rear seats up,undo the two bolts holding them in place.
Lift the seat of the anchor bolts, then out of the bongo (put it somewhere safe,not where your cat can sharpen his claws on them

)
Then undo the seat belt anchor point (its a good job i did this job,my passenger side rear belt was finger tight

)
Put all the bolts somewhere safe.
I hooked the hanging belt onto the ram for the tailgate.
Panel off now,simply get your fingers behind the inner lip, and pull and prise gently, yet firmly.
You'll hear the panel popping out of its anchors.
There you have access to the speakers!
Same applies to the other (driver's side) except,the rear panel fits into a lip on the middle panel (the one that houses the rear heater controls)
The rear panel can be extracated from the middle panel by a spot of manouvering up and down,and more carefull prying. Careful though!
The speakers are definintely worth replacing,just replacing those speakers without the amp/sub etc made all the difference
