In these contexts, your is most definitely not a vehicle of 'typifying generic force'. In fact, it is the opposite of that: the salesperson is trying to create in your mind an illusion that there is some sort of a special bond between you and the object that is offered for sale.
As has already been pointed out in more than one answer, one psychological mechanism of creating that bond would be to get you to think of the car as if you already owned it. By using your the salesperson may be trying to accomplish that, and thus weaken your resistance to buying the car. It is possible, however, that another mechanism is at work.
Consider ordering a hamburger in a restaurant. The waiter might ask 'How would you like your hamburger?' or 'What would you like on your hamburger?'. As you are waiting, the waiter may reassure you 'Your hamburger will be ready in a minute'. In bringing a bottle of ketchup to the table, the waiter may say 'Here is some ketchup for your hamburger'. In these cases, the use of your is entirely unproblematic, as there is a straightforward sense in which the hamburger is yours, even though it is not yet in front of you. Nevertheless, it should be noted that your is usually not necessary in these locutions, as it is obvious from the context which hamburger is referred to; the would have been sufficient. By using your, instead of the, the waiter brings to your attention, that the hamburger is being made specially for you, that care is being taken to ensure that it precisely satisfies your wishes.
The car salesperson's use of your may be an extension of the waiter's. Although the car is, obviously, not really built specially for you, the salesperson may be trying to create the impression that the designers anticipated your wishes, and have made the car in accordance with these wishes, as if they were making it specially for you. They knew, the salesperson would like you to think, that you have always wanted a rear-window heater that works in just this way, and, lo and behold, they have built the car with precisely such a rear-window heater. By referring to the features of the car as yours, the salesperson may be suggesting that they satisfy your wishes.
It is, however, unlikely that these intentions, or any that are articulated in other answers in this page, are consciously present in the mind of every particular salesperson who speaks that way. Many of them unthinkingly pick up that way of speaking from other salespersons, who picked it up from other salespersons, and so forth. Any answer to this question that may be given here, is thus bound to be a somewhat speculative attempt at a reconstruction of what may have been on the minds of whoever started the practice.