I restored an older radio recently for a friend with a 60s-era Mercedes. It could be set up for either positive or negative ground, but it was not a simple thing to do the conversion. I suspect that this was true of many radios made at that time.
Virtually all automobile radios before the mid 60s used vacuum tubes. They were large and heavy, and of course getting replacement tubes these days is a problem, especially since they used some strange types. The Mercedes radio still had its original Telefunken tubes, with European type numbering. I was able to cross-reference most of the types with American equivalents, but one couldn't be matched--there might have been no US equivalent. And this was a relatively common Blaupunkt radio.
From what I've seen on eBay, the nicer, period radios go for about $200 up. If you are willing to settle for AM only, they can be a lot cheaper, and of course if you are patient you might get a deal. On the other hand, there are modern radios made to look like older ones, and I've seen these on eBay too. They make a lot more sense, in my opinion. Also, a lot of 70s- or 80s-era radios are pretty generic looking, and not too expensive, so they would work well, I think. These are all transistor radios, not tube.
Also, if you are planning to install a modern radio, this is the time to convert to negative ground. You might find a positive-ground radio, but then what happens if you want to install one more electronic item, and it's not available in pos-ground? You then have to convert, and replace the radio you've just bought. Better to do the conversion when it first makes sense.