Does PSP9 have a 'smudge' tool like PhotoShop? If so, I recommend smudging the dirt streaks in the direction Garry pointed out, and if possible, adjust the 'strength' setting of the tool to something under 50%, so that it naturally 'trails off' when you smudge it.
It's funny, there are some fishing lures out here advertised under the tagline 'think like a fish' - I would say to pull off realistic weathering ' think like dirt'

and follow Garry's advice (i.e. what's the airflow, where are the strongest exhaust emissions from, and are there any areas more protected from slipstream where grease and oil stains may build up more?)
Don't forget too, to layer your weathering effects and adjust the opacity of each layer to suit your needs - that way you can do really grimy versions down the road by turning on more weathering layers and upping the opacity.