I’ll pipe up -It depends, my 2004 Ford Crown Victoria had all the wiring for cruise control, all I needed to do was add the servo under the hood and a steering wheel with cruise control switches. The more popular the option, the more likely it is that the wiring harness will have the wiring for it.
it’s a yes and no thing? for instance I added a tow controller, and that wiring was in place. The expected take rate is probably figured into wiring harness versions. And I’m pretty sure if I wanted factory upgraded bed lights, the plug is present in the trunk harness, and I’d just have to add the last 6’ or so. (I don’t, but I recall coming across it).