A fair bit of research led me to go the 2 double DIN route in order to update things. Having navigation available to me was really a big consideration (my passenger is dyslexic and hates giving directions), but I wasn't too keen on spending 20,000円 on a pre fabbed console. So the idea is to make my own from fibreglass. Honestly I haven't gotten that far yet as I found wiring things up to be a huge task in and of itself.

So, mine was one of the many with dash speakers, but no Clarion CD changer. The dash speakers are finicky and a bit ugly IMO, so I removed them entirely and wanted to run the front channels to the door speakers and the rear channels to the mini sub in the centre console. Or course all the drivers were shot, so I replaced the door speakers with standard 18cm speakers from Autobacs (maybe not so standard depending on where you live...). The sub was quite a trick, but I found this item that fits and works perfectly.

After the drivers were changed, the head unit needed to be wired up. I decided to use the factory Pioneer amp that fits in the sub enclosure, and no regrets there as this is all you need to power such a small driver. However, I quickly encountered a problem in that there were apparently NO wiring diagrams or pinouts available for the factory audio system whatsoever, so rather than solder everything in blind. I disassembled the Pioneer amp and Gathers head unit and took notes of everything scribed on their PCBs. From that, and some notes from guys that have a similar setup in the Honda Prelude I made these:

Pioneer amp pinout: http://imgur.com/a/6qh6B

Gathers head unit pinout: http://imgur.com/a/xPHdc

It might be pointless, but I figure someone out there will benefit from this data and not have to spend 2 days scratching their head like I did. As for me, the head unit install was a success and I now have a system that yields good, clear, loud sound from the door speakers and full, strong, resonant bass from the sub. The only drawback is a few songs with higher tinnier notes seem to break up slightly. I might be able to cure this by massaging my presets, but I guess a pair of tweeters might be needed.

I'll post pictures when I get the console worked out, as for now it's pretty ugly.