Is your converter made to charge a battery? That may be the reason it produces 16V.
Evidently some (but not all) automotive items can stand up to 16V. If you could drop merely 1 or 2 or 3V, then your led could receive a safe voltage. You'd need to install an led, or resistor that achieves such a voltage drop. Not easy to do.
If it were me I would turn a screw, or anything that looks like an adjustment. (And keep track of anything I do, so I can return everything to its original state.) Have caution because such tactics can ruin the converter.
Sometimes the adjustment is in the form of a fixed resistance which was installed at the factory. In other words, it's a component which affects the control section somewhere, but it can't easily be changed. If you can locate it, then you could experiment with its value, and see whether output voltage is affected.
Looking at your photographs, I notice the bare coils of wire, simply because it's accessible and substantial enough that it looks like it carries power, in a manner that adjusts power. As it turns red its resistance goes up, possibly performing a current-limiting function. So perhaps by shortening it you could cause output voltage to drop? Or would it cause output voltage to rise?
Suppose you were to clip a power resistor across its terminals? 100 ohms? 10 ohms? That would reduce its resistance. Would it affect output voltage? This is a tactic that could ruin the converter, so I should emphasize 'If it were me'.
Or else its resistance may need to go up, if you want output voltage to go down. This creates a challenge. It's merely speculation on my part, of course. You may get a reply with more helpful advice.