How do you clean a really dirty car?

I've just spent all afternoon trying to clean a RenaultScenic. This one is navy blue, non metallic paint, (sigh, regrets...). anyhow, the problem is that it has a full two years worth of road grime/bug splat/tar/diesel/kids snot/bubblegum/tree sap/pollen/old car wax/ear wax on the paintwork, and no matter what i use, i can't seem to really cut through the last layer of detritas and really shift it. I've tried Hot (very) Jet Wash, Zymol (crap). and umpteen different potions from Halfrauds, an electric buffer and it's It's just still covered in a manky goo and is driving me nuts, the Mrs is not bothered as it's just the daily battlebus, bought to be driven and eventually thrown away, but I want it clean this time, gleaming clean. Does anyone have any trade tips?, what about 'Cardealer', his motors must be all clean and shiny. Short of Napalming it, i'm at a loss.