Dirty windshields

I use Model master dullcoat.

I didn't mask the wiping area since I wanted to make it look like somebody had to wipe the windshield down with a towel because the wipers wouldnt do much without water, and they couldnt spare water often in North africa.

Don't overdo it.  I would be highly doubtful if the GAZ had a liquid wash resevoir -- this would mean that the clean area wouldn't be that clean.  The crew would take a dampened rag to clean the windshield properly.  Use your judgment on this rather than just doing what "looks" good.

I mask of the wiper area and then apply matt varnish to the windscreen, it gives that translucent effect of dust

You will need to scratchbuild a wiper, then use a compass to draw out a masking tape template corresponding to the arc of the wiper. Tape that down to the winshield, then use either a lightly misted airbrush coat, or pastel chalks to tint the window (airbrushing works better). Afterwards, remove the tape, and LIGHTLY smudge some colour following the arc of the wiper.

thanks for the tips, this is the first time I've ever done any vehicle with a windshield.

Yes Rob the Gaz 67B definitely had a wiper. If you look at the museum photos at the top of the Tamiya instruction shhet, you will see that the drviers side had a wiper. However, the wiper was oriented from the top. I just finished my Gaz and was considering the same thing. To weather or not to weather that is the modelers question.  

I,ve always used a crescent of masking tape, used to shield the area that would be swept by the wipers.Then i used my airbrush to apply a light coat of dust.