Can this also mean 'we used to listen to the radio' (past imperfect), or is it strictly past tense?
I you want to express a certain habit in the past ("we used to...") , it's better to add something in Dutch if it is not clear from the context. In this sentence, I would say: "we luisterden altijd naar de radio (more appropriate for speech, I'd say), or "gewoonlijk luisterden we naar de radio" (which I'd pick for written language).
Without context I would not interpret "We luisterden naar de radio" as a habit, but rather as something that happened once in the past or as the background of a story.
Hope that helps.
Thank you, that helps a lot, since DL focuses mostly on direct (and literal) translation and not as much on context which can make a big difference. I was just wondering if we would still use the same 'luisterden' conjugation for both tenses, which you have also answered.