Hmm. Of course now I can't find my original source. :/ According to this article
https://ranwhenparked.net/2013/07/22/30-years-ago-bmw-launch... Mercedes' and BMW's interest in car diesel engines goes back to the 1970s, though Peugeot was there too.
https://en.wikipedia.org/w/index.php?title=History_of_the_di... is oddly written and fairly uncited, but it does state "The diesel car markets are the same ones who pioneered various developments (Mercedes-Benz, BMW, Peugeot/Citroën, Fiat, Alfa Romeo, Volkswagen Group)".
Moving towards the politics, Rüdiger K. W. Wurzel's Environmental policy-making in Britain, Germany and the European Union's Chapter 5, "The catalytic converter versus the lean-burn engine controversy" seems very informative. Particularly the section "The diesel engine", which contains this:
"In America and Japan, diesel cars almost disappeared from the market in the 1970s after the adoption of stringent limit values for carcinogenic particulates, although this was disputed by the car industry. The EU introduced relatively lenient particulate limit values in the late 1980s. The German government demanded the [Best Available Technology] option for both petrol and diesel cars. However, after heavy lobbying from Mercedes and Volkswagen, fiscal incentives were granted not only for the three-way catalytic converter but for diesel cars which met the lenient EU particulate limits."
(Seen through Google Books at https://books.google.com/books?isbn=0719073340 )
Industry and Politics in West Germany: Toward the Third Republic by Peter J. Katzenstein (seen at https://books.google.com/books?isbn=0801495954 ) adds
"Moreover, since diesel engines get the same break, the European regulation also favours those German producers who are leaders in diesel technology, especially Daimler Benz and VW."
However, Wurzel clearly asserts that the move towards US-style catalytic converters on petrol cars in Europe (which is apparently what first made diesels attractive as a regulatory end-run) was initiated by Germany in the teeth of opposition from German car makers (especially Volkswagen): there was a perception in the UK government that Germany was obliging its automakers in this, but it was wrong. Also, http://enveurope.springeropen.com/articles/10.1186/2190-4715... says that tax incentives for dieselisation in the domestic German market are relatively modest (and that there was a small tax penalty for buying a diesel until 2009, though not after actual fuel costs were taken into account) (2.3.6, 2.3.7). Interestingly, it also suggests that a supply glut of fuel oil (which can be converted into diesel) was the original incentive for the European diesel boom (2.3.1) . Then again, further on down the road it seems clear that the EU's lenient approach to diesel emissions had the effect of advantaging European automakers collectively against competition from the Japanese, who had bet on hybrids instead (2.3.6).
In summary, TIL.