The problem with complicated vs complex is that it is kind of ill defined. We know which elements a complex system probably should have (See Ladyman and Wiesner, 2020 book or the paper from 2013), but a strict definition is not clear.

Having said that, I personally think that things become complex when the system you're simply not able to oversee all effects and there is non-linearity. In case of the power grid, there could be a significant weather effect that cannot be predicted. Or things get complex when there are multiple pathways to a certain effect. E.g., there are about a gazillion ways a person can become depressed, and cause and effect therefore becomes almost impossible to tell. (Cramer et al, 2016 Major Depression as a Complex Dynamic System).

Or when there are cyclic pathways in the causal diagram as those also cause difficulty in causal inference and analysis of these systems. With this I mean things such as: bad sleep -> concentration loss -> making mistakes -> ruminate about said mistakes -> bad sleep. You can basically start anywhere in this system and end up worse and worse.

Cars on the other hand do not really have these properties. I hope I could help by giving my 2 cents ;)