I would address this the same way I would address this with a child (even though it's the reverse here). Sit down, have a conversation, where you discuss with them your feelings, why you want what you want, ask them why they do what they do, and then see if you can find a middle ground.
After all, your solution (salt less in the cooked food then individual salt to taste) seems eminently reasonable, though I don't know the specifics. Why won't they agree to that? Find out the reason, and try to approach it non-judgmentally and non-argumentatively. Just like your parents should take you seriously when trying to find out why you want/choose to do something they would like you not to do, do the same for them.
And when you explain your side of things to them, I strongly suggest sticking to your preferences and health only, and not suggesting they cut back on their own; might be appropriate also, might not be, but better left for another time. You'll get more resistance if they think you're trying to tell them how to live, same as you'd resist them if they were telling you how to live.
Once you understand why they salt the food so much (and why they won't stop), and they understand why you want them to salt it less, perhaps you'll both find a solution.
Perhaps that solution is cooking food that doesn't require so much salt (Curry, Spaghetti, things with strongly flavored sauces for example). Perhaps the solution is changing some food to have less salt and not others. Perhaps it's changing to a different kind of salt - Koshering salt you can usually use substantially less (same volume, but bigger crystals means much less actual salt by weight), for example, and still have the taste impact.
And perhaps in some cases they have reasons for using salt the way they do. In the culinary world, salt has some very useful effects that are important for certain dishes. Well salted eggplant or zucchini, for example, is important when baking in order to draw the water out (the salt can be removed afterwards with a towel, in some cases). Salting a steak before cooking is important if you like a crispy skin on it. There might still be a middle ground (where some, but less, salt is used, or again koshering salt or similar large crystal salt), or you might be able to stomach more salt on things it's needed on if you get less on others - who knows. But you'll never know unless you talk about it and find out their reasoning.
Finally, if you do have a long conversation about it, it may well stick in their heads more effectively; perhaps the issue here is simply memory and old habits. Include, perhaps, a discussion of what they can do to remember to include less salt. Perhaps they salt by the 'pinch' method or use a shaker; perhaps they could switch to using teaspoons and measure it properly. Try to think of a few suggestions along these lines that are actionable (i.e., things they can specifically do, not just general opinions). This sort of thing is very useful in the business world once you get there, as well; learning how to not only convince people to do what you want them to, but learning how to get them to do it as well, and being action-oriented.
