I join in the question: “Where did they all go?” These were pretty common in the upper midwest (as imports went) in the late 60s-mid 70s. I knew a Lutheran minister who owned one (maybe a 69 or so?) in the mid 70s, and got to ride in it once. He seemed pretty proud of the fact that he had put about 80K on it and it seemed to be running strongly. Although it was starting to look a little worn, it seemed to hold up a lot better than my Scoutmaster’s 69 Cortina.
I bought a used Opel Wagon in 1971. It came to the US from Brazil where my college roommate’s father worked for an American firm. It was a 1968 model, had impressive handling and was fun to drive. While the car was excellent in all respects, and I owned it from ’71 through ’74, I took a lot of grief about it. Gas station people made comments like “Is this your car? It’s not paying your taxes.” and “Does this thing come with a motor?” and “This thing is made for insects.” But, worse, were the insults from the Queens County (NYC) Buick dealer parts department who consistently said really nasty things to folks buying replacement parts: “Ya look like the kinda’ guy who’d buy this ting”. After completing my Industrial Design degree, and I went on to design vehicles here and abroad, I never bought a GM car.