If you got your car out of the country with a temporary residency DNI, I'd say you were lucky. If you are a foreign resident of Argentina, the law is you're supposed to have permanent residency to take a car out of Argentina. Particularly with a letter and an expired DNI I wouldn't think you'd have as much luck, but then you never know.
I have gotten my car across the border at Formosa, by sheer luck and persistence, when I had a precaria for permanent residency. Previous to having the precaria (i.e., no residence, passport only), I'd traveled a number of times back and forth between here and Paraguay with no problems but that all changed the moment I applied for residency. you are supposed to have to have either a citizen DNI or a permanent residency DNI.
I feel that I was extremely lucky to get across that one time - I tried everything I could think of at the main border crossing in Formosa (including talking with some people about a bribe - hey, we'd traveled a bit over 15 hours straight and didn't want to think about turning around!) and there was zero probability of getting that car across. One guy did tell us we might try the ferry crossing down the road a few kilometers because they were more lax there. And indeed, it was a small border station, but at first they didn't let us either, until we begged and pleaded and the girls started crying! And that ferry - my god. Flat barge, big, with 18 wheelers rolled up on it and vehicles carefully balanced (hehe) to ensure the barge didn't tip over in mid-river. Phew.
It would be nice to know of a place to go to cross where you can do it regularly. But even if you have done it regularly, I wouldn't necessarily count on always being able to do that. you're less likely to have luck with an expired DNI and and papers from immigration. I'd probably go to customs before you go traveling and see what they think and weigh your probabilities.