Insight into the electronic structure of conjugated polymers used for organic electronics applications is of outstanding importance. Time-resolved electron paramagnetic resonance spectroscopy of light-induced triplet excitons provides access to the electronic structure with molecular resolution. Systematically investigating building blocks of increasing length and comparing the results with the polymer deepens our understanding of the structure-function relationship in organic semiconductors. Applying this approach to the copolymer poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) known for its efficiency and device stability reveals the electronic structure of the polymer as well as each of the smaller building blocks to be dominated entirely by the TBT moiety. Hence, the usual description of PCDTBT as a carbazole derivative is somewhat misleading. Furthermore, delocalization extends along the backbone, over at least two repeat units, and is consistent for singlet and triplet excitons, quite in contrast to other push-pull systems previously investigated. DFT calculations of the spin density distribution agree well with the experimental results and show the BP86 functional to be superior to B3LYP in the given context. The polymer and all its building blocks show a remarkable homogeneity that by ruling out aggregation phenomena is ascribed to a rather rigid and planar backbone geometry.