Once the plastic panels start to fade I would say the best bet is hit them with a fine grit sand paper and repaint them using spray paint that works on plastic,vinyl ect. Most panels come apart pretty easy as long as you ain't too worried about taking your interior apart (which drives my father nuts :P) The best thing I've used on keep my interior clean is a protectant every couple of months (not armor all cause it attracts to much dust for my liking) but I've had good luck with house hold degreasers for getting real stubburn stuff out, but it damages the panels a bit if you repeatily use it, another thing that works well is straight hand cleaner it gets into all the little cracks and works the grime free, using a hand brush also helps alot. With about an hours worth of scrubbing you'd be surprised how your interior looks after you add a protectant