Alan, I used to work at a sound system installation shop for 5 years. I did sooooooooo many installations and modifications for sound systems as well as numerous cars for competitions.
Needless to say, I wanted the best possible sound for my 33 as well. The front speakers are a bit tricky and needs a lot of work and patience into it.
I've put DLS Ultimate Pro component system speakers in mine. 6 inch on front doors and 6 inch on rear doors.
If you convert your rear doors into power windows like I did, All you'll need are a couple of spacers to keep the speakers from hitting the rear windows.
For the front is a different story.
To begin with , there are two versions of power window mechs for the 33. One of them has the motor looking down towards the lower front corner of the door, and the other one has the motor facing in the exact opposite direction.
Both of them fit all 33's ( S3's ) but you will need the second one so you can have the clearance in the front lower corner of the door.
Then you need to cut a section your door cards the size of the speaker mount.
Put your door card back on the door without a speaker on it, and mark the area of the metal you need to cut off.
Depending on your speakers, you will need to calculate the size of the spacers you'll need to make.
Then for the tricky part.
The only place you can place the speakers on the door is unfortunately between the lower plastic part of the door card and the middle section where the cloth of the door card is. If you put the speaker without any mods it wont sit properly, you will loose sound from the gap and it will look ugly !
You have two options here.
1 - shape the spacer in order to sit nicely into the 2mm gap
2 - Cut a very thin ply of wood and shape it to the door card so that your door card is completely straight , like I did, and glue it, bolt it onto the door card.
Then after you dress them up and all, all you have to do is pass the speaker cables through the plastic round tracking that goes into the door and where the power windows and central locking cables pass through.
Thats also a bit tricky , especially if you dont have patience and thin fingers. I threw a lot of cutters and screwdrivers as far as I could from my frustration during this process
Then you can mount your speakers on