6 is an evil number, and 6666 is a super evil number, so why was this set, an ambulance, given such a number?
Interesting torso they chose for the woman, one of the more revealing outfits that she has. Perhaps she is a young college student who got alcohol poisoning after using her fake ID to get into the bar? Something of note are the incredibly small wheels on this vehicle, not something you see very much today. Additionally, this set marks a shift in how medical sets were designed. If you look at the 1981 ambulance...
...a red cross is used instead of the Star of Life. The Star of Life was designed in the United States 1970s, after the Red Cross organization complained that the current United States medical symbol was too similar to theirs. 6666 was only released in the United States, while 6680 was an international release. So, we can see that by the 1990s The Lego Group was changing their models or at least designing models specifically for certain countries, rather than using the ubiquitous and universal Red Cross symbol.
It's also interesting to look at the designs of these two ambulances. Both are very similar in many ways, both in terms of construction and how they can be interacted with. The main difference is how the back opens, as you can see in the photos.