My mom and dad have a vintage Bronco that's worth more than they bought it for decades ago. They have people approaching them a few times per year asking if they might want to sell.You throw out an example of something you think should follow a rule of always depreciating and I can find you an exception where something in that classification has increased in value.
Older small trucks have become increasingly desirable because everybody stopped making truly small trucks. 80s era Ford Rangers are actually quite desirable now. The older square body pickup trucks of all sizes are pretty desirable right now. Their values have changed, and quite possibly increased. And yeah, old car head units with a functional 8 track might fetch a high price on the collector's market for people who are doing restorations.