On top of what RedFish and 67Dart273 said, I would just look at the schematic and use relays in the same places that the factory used them in. There are only a few used unless you had the fancy add ons like power seats, windows etc.
Most of us only use them to compensate for voltage loss or for aftermarket add ons that would require running large gauge wires all over the place, like RedFish mentioned. Sounds like you've got that figured out with your trunk mounted battery.
I'd take into account that relays add another failure point in the circuit so I wouldn't add them unless absolutely necessary. Wires and quality splices don't normally fail randomly but I've seen a lot of relays fail. The standard headlight switch connectors don't normally have issues under stock application. They potentially can if you start adding fancy lighting that draws more current than the switch/wiring was designed for. If you think you're adding more than what the factory did on those circuits, then I would consider using relays to keep some of the load off the switch.
