Having gone through all the scripts using key word search terms, I decided that they were in the nacelles. That said, there were clearly other forms of reactors for power and batteries throughout the ship. I have no problem with that.
I also have no issue with other Starfleet ships have the main warp power
reactors being located in the main hull(s) as per design requirements. But I'm completely convinced that Starship Class (Constitution Class) ships, i.e. TOS Enterprise and her sisters, all had their main warp reactors contained within the nacelles.
A little off topic, I even really think TMP's main reactors should be a more efficient stream lined version of TOS's systems. Where the power from the nacelle reactors are the sole source (Other than backup batteries), and engineering is where that energy is converted and shunted to the rest of the ship. Minus other reactors like in TOS. Then by the end of the 2200's we see a conversion to a centralized warp core that are due to more efficient and powerful controls for m/arc energy containment. Pre-TOS had a centralized warp core because they weren't as powerful, and required more hands on maintenance.
Just my two cents to explain the variations on the same ideas.