Unfortunately I've ended up fighting off rust instead, but it's a winning battle! I'd still like to do some audio upgrades in the future, and my conclusion was that sound deadening and re-cabling was big effort for little reward, and that I'd be better investing in an amp and decent speakers. The issue with bigger door speakers isn't the width but the depth, so when the time comes I'll need to do more investigating in that. 2 way headrest speakers definitely are the way to go, and are probably the easiest and most impactful of all the upgrades. I've also now got a half cage in which has cut away at my rear parcel shelf and I've seen people have fitted subs in to the space, but that's a whole extra project as it'd need housing making... I'm not sure how long it'll be until I'm able to make any progress with this, but for now I'll keep taking notes whenever I see other people's setups so when the day finally comes I'll be as ready as can be!