Ok. The Germans bring in a handful poorly trained spys that have been given radios, explosives, and other sabotage equipment either by airship (dirigibles are still flying in 1936) or passenger ship. As the Germans are woefully ill-informed as to what constitutes American "war industries" at the time, and most industries that will eventually be major war producers are currently engaged in solely civilian market production (eg., Ford, General Motors, Balwin Locomotive, Kaiser Aluminum, etc.) the spys are left to their own devices to attempt to identify suitable targets.
This they do attempt in crude ways such as asking locals about various plants, and that sort of thing. They focus almost entirely on the Northeastern US in states like New York and Pennslyvania having no real conceptual understanding of the size of the US. They also lack the funds and transportation to really move feely about the nation.
Within a few weeks to a month or two, one or more of these spys screws up and get caught for some reason or another. The FBI quickly becomes aware of the plot (just as they historically did in the one German attempt).
This leads to two things: The arrest of the other persons involved. A diplomatic disaster for Germany.
The US public opinion that varied from supporting Hitler and the Nazis, to indifference, to mild distaste for the regieme is now heavily solidified against the Germans. Hitler is denounced as a criminal assassin, hater, etc. The US government announces embargos of critical materials to Germany and begins to press US corporations to divest themselves of their German holdings.
The long term result is that the two biggest auto manufacturers in Germany; Ford and General Motors (Opal) fall in size and scope leaving the German auto industry a fraction of its historical size. This in turn forces virtually a demotorization of the military. The aircraft industry is hit hard for alloying agents that the US provided. This retards development. The chemical and petroleum industries are badly crippled by the embargo forcing the Germans to spend considerably more on development of these critical industries.
The embargo in turn retards military investment and limits the size of the German military expansion. By 1939 even though Hitler has still greatly expanded the military and made many of his political gains that were historically done he is facing a more thoroughly equipped Britain and France due to the US being far more willing to provide military materials. In addition, he is in poor shape to invade Poland or force a military solution to his demands due to having a smaller mechanized force. This forces either a delay in invading Poland or the invasion takes much more effort and is far more costly. This in turn delays the various operations in the West sufficently to end those in a stalemate rather than a lightining victory.
Germany faces political problems at home as Hitler is now accused of starting another World War One.

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Truth is stranger than bullshit!