You can always check your progress in the rebel drop menu. Vehicles are diverted in diffrent classes. Vehicles are located by delivering them to Marios garages.
Military vehicles do not count (you will also be told so in game)
If you are in or near a settlement which you already have unlocked the local garage (by liberating the town) and you enter a car which you have not collected yet, the closest garage will be indicated by a target symbol in your HUD.
Also I strongly advise you to activate and use the "Collectors" mod.
Also
FLYxBOII has posted some videos with the locations of some rare vehicles in his solution for the XB1 pendant of this trophy
HERE:The last vehicle I was missing was the civil version of the "Weimaraner" SUV, the "Weimeraner W3" wich I could not find anywhere. Then I looked up on the
Just Cause Wikia to find the location. Two are at N 40 48.991 E 5 37.956. On the southern island in the north bay of Insula Striate, between Maestrale and Grande Pastura. (may only appear at night)
As always: Please leave a comment or suggestions if you think somethink could be added or is unclear. Anonymus downvotes of solutions are not fair to the author.