The smartest thing to do is follow the road up to the radio tower garrison. The road shouldn't be well guarded. I only found a few groups of 2 Germans on the road and a single machine gun position covering the end of the walk. This is another fairly easy base to raid. Just push inside and use the shotgun to clear the path. An important thing to remember is that you don't have to clear out the entire base. Just kill a few of the soldiers and get to the point at the back of the base. Plant the explosives and sprint over to the next spot. Then rush by the tents and plant the

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last one while you crouch behind cover. Dexter will obiliterate the base in 15 seconds, so you just have to run away and wait.
The last base to tag is the munitions base near Dexter's starting point. I leave this for last, since it's a bit of a tough nut to crack and there isn't much to gain from disabling it early. If you are really low on ammo, you could try and resupply at the box by Dexter. I think it'd be much easier to just pick up a German rifle though.
The munitions depot is well defended. There are several patrols around the base and a few machine gun positions guarding the entrance. You'll need to quickly snipe the gunners if you want to survive this fight for long. Once you get inside, it's just a matter of using the tanks for cover and picking off the men guarding the entrance into the base. Plant your tags and leave the last one inside the barb wire cage. You can just jump out over the machine gun positions in back to get away with time to spare.
Once all 3 bases are destroyed, it's just a matter of reporting back to Dexter to see that things are changing a little bit.