Several times throughout the episode, the 1979 Ford LTD driven by the antagonists changes into a 1979 Mercury Marquis. This can be seen starting when Magnum is watching it while it's following him in the rear view mirror, and clearly at 22:45 when the antagonists drive by.
When hijacked, Rick is told to "follow that (black) car", but in later aerial shots from T.C. helicopter, the black car is behind Rick's white van.
TC's helicopter used to pick up Magnum, Sharon and the dog (approximately 40 minutes) is a different one than from the beginning. Although the registration is the same (N58243) this one has slender skids with no covering and the earlier type "V" tail instead of the "T" tail featured on later versions of the MD/Hughes 500.