In classical differential geometry the integration morphism intergal: omega-(p)(M) --> S(p)(M) is a morphism of complexes inducing an isomorphism in homology. In the synthetic case the two complexes are in fact isomorphic. We exhibit a cochain morphism D: S(p)(M) --> omega-(p)(M), which is an inverse for integral.
Félix, Y., & Lavendhomme, R. (1990). On Derham Theorem in Synthetic Differential Geometry. Journal of Pure and Applied Algebra, 69(1), 21-31. https://doi.org/10.1016/0022-4049(90)90075-S (Original work published 1990)