Perhaps the images did not strike you as consensual, as evidenced by the stick figure's positions. Remember the old twist on the "Hang Ten" feet, where a smaller pair of feet was inverted and between the larger feet? It was considered a bit racy at the time, but it was also kind of cute. It didn't have the menace that you felt. Oh, yeah, maybe that's why they call them "triggers".
I agree that feeding the troll might not help. Is this a Company parking lot? If so, maybe think about looping HR in, maybe?? Otherwise, doing nothing, beyond counting the number of stickers to estimate the owner's IQ and EQ, is probably the best course of action.
I studied it a little closer this morning to try to determine what was bugging me about it. The image shows the female stick figure with eyebrows raised and her mouth in an "O", which appears to be shock or surprise. Her hair is also disheveled. The male stick figure is winking.
I work on a military base, but there are few military people here. I'm a contractor, so there isn't really an HR for me. This is a strange, creative, and scientific environment where you're pretty much left to yourself and there's really no one to "tell".
I feel angry because the owner of this truck is clearly making an aggressive statement, but using a cartoon do so, thus pretending it means nothing. It is violence against women. There's no doubt in my mind. But I am sure that if I were to say that publicly, then some other people would gaslight me into thinking "huh, how can stick figures even have sex?" I'm also angry because in this day and age, I have to accept that some asshole is walking around thinking that violence against women is funny, and there's either nothing I can do about it, or nothing I can do without fear of reprisal. It's 2019, why are we still having this conversation?
I cannot help but wonder if you and the owner are viewing this graphic differently. (I still find it in poor taste) I grew up with a group that would find that sticker funny and more of a sarcastic jab at the folks who have stick figure families on their grocery getter or meant to insult those who hold different values than the ones that they have (the same crowd tends to love the calvin urinating on the logo of a different truck brand).
The folks I grew up with wouldn't see any violence against women in the image you describe, but instead would view it as bragging that women prefer "real men" (which they happen to define with all the characteristic the believe they have). They also would interpret women as being attracted to the values they hold (woman "humping" the NRA/pro-gun logo). The only offensive message they would see, and intend, in these sticker is aimed firmly at the people who disagree with them; basically calling you (the disagreeing viewer) less than a real man who members of the opposite sex have no interest in. The Lincoln with guns . . . well I don't know, maybe they just find it cool?
Honestly nothing in your description so far, other than your interpretation, scream rape/violence; not a childish wink when dealing with the topic of sex, not the choice of position, not the woman having an "O face", and not the fact that a woman's hair might become disheveled during the act of sex.
I went so far as to look online for a source of a sticker similar to what you describe (trigger warning: there are stick people engaged in sex, well behind a censor bar, in the following link) and the closest thing I could find is this:
https://www.amazon.com/stick-Figure-Families-sticker-censored/dp/B00CFUFM56. Which I simply do not see violence/rape in.
I do not encourage you to engage or spend any of your time thinking any further about this an I certainly do not agree with a need to photograph this item. If it were me, I would try and convince myself that the owner had attributed a different meaning to the images then the one you have, try and put it out of my mind, and go on with my life. Letting it stew in your mind is likely part of the intention of the owner and, based upon the folks I grew up with, any action on your part could be seen as permission to escalate. Again based on the folks I grew up around, if you edit or add stickers or banners to their truck (as suggested in this thread) you better be prepared to find stickers on your car that support positions you disagree with or are simply meant to be offensive.