:)
Revenge of the Desert Phantom first mentions and describes how Frank and Joe get a used blue police van. (#84 in The Hardy Boys Digests). Dead on Target (#1 in The Hardy Boys Casefiles) first mentions and describes how Frank and Joe get a used black police van. There are two different stories within the continuity (and two very different reasons for why Frank and Joe need a new car), but those would be the two to consult. (And it makes sense that #84 in The Hardy Boys Digests would mention it, as this title is considered a prototype for the Casefiles.) It's interesting that this is one aspect that both the Digests and Casefiles maintained in common, from #84 through the end of the series (#190 Motocross Madness).
And the Hardys eventually get another van after their original black van is blown-up (seeing a pattern here?) in True Thriller.
Thanks for that extra tidbit, SDLAgent- I'd forgotten about that. But it's been a while since I read True Thriller - that was a pretty good one- I think it may have been one of the last to feature The Asassins too.
I believe the last book to feature the Assassins was #108 Blown Away.
Oh yeah- that's right, they were in there too. Of course, I think the Assassins appeared in some of the Supermysteries as well.