Some maps of the final free-and-fair Reichstag election:
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To make further sense of these, some demographic information:
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All sorts of things can be noticed and commented on here, but with respect to the late stage rump DNVP:
1. While we tend to associate it with the countryside, and in particular with the long lost world of rural East Elbia, the party retained a degree of support even until the last amongst bourgeois voters in the cities and commuter suburbs. In Berlin, for instance, they always polled a solid share in Zehlendorf, larger than in some of their rural strongholds.
2. Which is not say that the association with rural Lutheranism and large estates is inaccurate: observe that they polled best in overwhelmingly Protestant regions with high proportions employed in agriculture and low or average proportions of the workforce graded as 'family workers'. It is just that this association is not the whole story.
3. A high proportion of the families still backing the DNVP at the end were on the wrong side of the Iron Curtain and a high proportion of those were on the wrong side of the Oder-Neisse line. Refugees ended up all over the place, but those from regions closer to the Baltic were more likely to settle in the north of the new Federal Republic. In particular a lot of East Prussian families settled in Schleswig-Holstein. Of course, once one adjusts for the part of the Hamburg metropolitan area in the latter during the 1920s and 30s, voting patterns in East Prussia and Schleswig-Holstein were actually very similar during the last years of Weimar.
4. It is dubious, anyway, that it is even possible to disentangle DNVP supporters who stayed loyal to their old political party (the minority) and those (the majority) who switched to the Nazis. There's some evidence that age may have been a factor, with the elderly more likely to stay loyal. Which further reduces the number of ex-DNVP voters around by the end of the 1940s. A fundamental issue is that while the DNVP was not part of the Völkisch political tradition, it did incorporate Völkisch elements (and right from the start) into its appeal and propaganda, meaning that the boundary between the two camps was paper-thin. It, of course, why the party and its political heritage was considered beyond the pale after the War, but it is also one of the main reasons why disentangling everything is impossible.
Hugenberg's right-shifted rump DNVP was arguably the vehicle of an elite that always had been rather anti-republican, but which from 1928 on wanted to finally put an end to the republic once and for all. This is reflected in its strongholds being among bourgeois Wilhelminian elites and in areas where the old Junkers still had the most sway.
The rump DNVP's ideology was in many aspects similar to the NSDAP's. The NSDAP was just much more successful in attracting voters across the board, ironically often voters than had left the DNVP because of its shift to the right. In some senses the NSDAP turned out to be the big vote-attraction machine that anti-republican big money had wanted Hugenberg's DNVP to be.
Speaking of which, what happened to the nazi voters post-Weimar?
Even in the free and fair elections, the NSDAP still got 33% in 1932 and even in 1930 it had 18% of the vote. That is a significant amount of people
That is of course a very good question. In fact as mountvernon (the OP) said the SPD and KPD voters after 1945 would mostly go to the SPD, the Center voters to the CDU and the BVP voters to the CSU (CDU in Palatinate). This leaves a bunch of DNVP voters that already have been discussed, and many, many NSDAP voters. By pure arithmetics a majority of them would go to the CDU/CSU, with minorities going to SPD, FDP, other parties and abstention. Most other parties had already lost most of their voters to the NSDAP by 1932/33
In fact if you look at electoral data over a long timespan you see that the Catholic camp always remained very stable. The socialist camp (with SPD, USPD and KPD seen as communicating vessels) was mostly stable from 1912 on, apart from the 1919/20 peak that it was not able to hold. The rump DNVP held on relatively well among some milieus, but had an electoral breakdown in most areas. All other parties never managed to build up loyal supporting milieus. A typical voter biography for a voter born in 1890 might have been: 1912 Nation Liberal, 1919 DDP, 1920 DVP, 1924 DNVP, 1928/30 Economic Party, 1932/33 NSDAP, 1949+ CDU.
Finally while probably a majority of NSDAP votes came from non-Catholic non-socialist parties, a very significant part came from abstention or first-time voters. It might be more tricky to tell where these went after the war.
