I just let people take whatever save they want. I personally believe it should be a 4+ for everyone not just vehicles even if the rules dance around the subject and never explicitly say it. Their are at least a dozen places in the book where it is implied to some degree that it works that way. (same **** with FMC vs Flyers against blast weapons.)
But if my opponent ever said I only get a 5+ for being obscured by a ruin i'd go with the 5+ without arguing it because technically RAW its true. That and I never expect to ever have cover saves if I am not toe in cover and it is just kind of a bonus if it happens.