Installation:
Copy the dll and ini file into your scripts folder

The ini file has two sections:

[DIRTY ZONES]
This section defines zones that you want to class as extra-dirty. Any zones in this section will add the DIRT BIAS value on top of any dirt settings.
All entries are defined as "ZONE=[zone name]". The zone name must match the zones in the game to be recognised.

[SETTINGS]
SEARCH RANGE - How many units away the mod will collect cars from. The further away, the more it will collect and the less chance of seeing them change their dirt values while they are in front of you. Beware of setting this too high as it might affect performance, although I haven't seen that occur on my PC.
Default value = 100

DIRT BIAS - How much extra will be added to the dirt values in areas that are defined in the DIRTY ZONES section above.

CHECK RATE - The number of frames between the surrounding cars being collected and checked. Setting this too low with the SEARCH RANGE too high could impact performance. A value of 1 = check every frame, default value is 4

Version 1.1
Now building an ID string from Vehicle Name + Vehicle Class + Number Plate + Number Plate Type + Primary Colour + Secondary Colour, to minimise any chances of cars getting caught with a repeated number plate

Version 1.2
Minor optimisation on car collection handling
Add new option in the ini file to control how often vehicles are collected and checked