An abandoned vehicle is a stopped vehicle, it isn't a wreck, it is a vehicle. D6.4 is phrased in a way that permits becoming a passenger.
On a related point, a vehicle is covered with a prep fire counter. It is a stopped vehicle. Can passengers load onto a vehicle that has a prep fire counter? D6.4 says stopped vehicle that has not moved. The vehicle with a prep fire counter meets the literal statement of D6.4.
How about a motion vehicle that expended a stop MP but never "moved" during that turn. Can infantry load into that vehicle and become a passenger? C.8 tells us that the motion now stopped vehicle is a moving target even if it did not enter a new hex in the player turn ... but has it "moved?"
If the best cover in the area is an abandoned ht, a squad can become its passenger if for no other reason than it is better cover.