Although I might be wrong, I assume that German used to be rhotic (simply said, just like British English used to, and American English still is) because there are still some rhotic German dialects today. By rhotic, I mean that each letter R that is spelled in a word is always fully pronounced as a consonant (not omitted or turned to a vowel).
The history of English on this topic is quite well documented, but I can't find anything about it for German. So was German really rhotic in the past? If so, when and why did it stop being? If not, how did rhotic German dialects develop?