To tie in with all those
"they have continued to communicate with earth"
"they left sleeper cells behind"
ideas, here is one how communication may have worked in that time:
Ever heard of "Number Stations" ? -> Read about it : http://en.wikipedia.org/wiki/Numbers_station
For Short: There are radio transmitter stations that do send -read out- numbers that seem to be some kind of communication throughout the real world of today and in the past. Although they are thought to be for spy activities they did NOT stop after the soviet breakdown. Indeed activity seems to have risen since then. For the most part these can be recieved by simple broadcast radio receivers.
Nazis that went to the moon in 1945 most probably did NOT have some sort of sophisticated radio or computing equipment with them. So they have no Internet up there. But simple radio seems plausible. That would also concur with the giant wrench and the krad seen on the first two tariles. (Heavy machinery, nothing too modern)
Maybe these number stations are what they continue to use to communicate with their earth-bound allies, spies or sleeper cells.
Maybe they have knowledge of what happens down on earth, but are unable to fly down to acquire modern technology, like an battery drill :P
Maybe that we will see many more "Numer stations" in the coming years and 2018 will be their peak, in which they signal the spacenazi's to get back.
Sorry if this idea has been discussed before, we need a wiki...