Have look at Bump Automorphic representations. The adelic picture is given there as does Tate's Thesis. Although Tate's proof works as well in characteristic p, he assumes the number field setting right from the start. It is simply an exercise there and Tate actually talks about Riemann Roch as an equivariant version of Poisson summation. Perhaps, also Weil's basic number theory is worth a look.
Marc Palm
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