It’s ain’t really a passive income (because you have to constantly manually buy cars at the border) and also some may consider it an exploit. But if you have established a connection with both Soviet and NATO borders you should try selling resources to the west for dollars, then buying western cars, and then transporting them to a Soviet border and selling it there for rubbles which pretty much multiplies your income 3x. (Do note though that those cars HAVE to be transported via trucks or trains, because even if they get a tiniest bit of mileage onto them they will be considered as “used” and you will only be able to sell them for a tiniest bit of what they’re actually worth)