There are three mindfields. you have just go as far you can north and walk your dude down to the roadblock with the mine sweeper. once you detect mines, you have to sweep the whole thing right to left. The guy will make a comment like field is clear.

How do you know if the guy has detected mines? I can never seem to find them. It's the only secondary objective I can't complete. I can see the guy has the mine sweeper out. do I just move the guy from north to south and that's it?

By the way, I never see engineers putting mines in the road. But I do get the message indicating that I should look for mines when I approach the road and it shows me where to look (red arrows).

Those M18's are Flares used to mark a spot for artillery. A Commissar should say something about a truck behind the lines with flare to make artillery targets at the start of the 2nd part.

Just got into this mission and been having trouble on completing 2 of the secondary missions, firstly ive placed all the timed explosives on the tanks in base but for some reason its showing as objective failed, also with the radio transmitter.. how your supposed to destroy that?

Tried to look some walkthroughs but none helped...

OMFG, I spent like 10 hours trying to figure that friggin radio transmitter thing out........450,000 fails later I just gave up on that and crossed the defense line and vowed never to play that mission again
or alteast I dont try to do that objective again

It was quite simple had to sacrafice 1 man to nade the radio.. dont know if there is any silent way to destroy it