Reduce dirt on windshield - which png?

[Medway]

Hi I'm trying to reduce the dirty windshield effect by editing the Glass NML files (I did this with an Aerobask plane so figured it might work here too). So far I've done 4 of them but they don't seem to be affecting this. Is there a PNG I can edit the transparency of to reduce it (it's more noticeable now in V12).

 

Thanks