Blackout relays

If i cut the headlight wire, and one side to 87a and one side to 30, 86 to a toggle switch with 12v on the other side of the switch, and 85 to ground, would that work to switch my headlights on and off, as long as the headlights were turned on at the stock switch? Then would it also work to branch off between the toggle and the 86 on my headlight relay and connect to the 86 on another relay hooked to brake lights in a similar way? does this even make sense?

yep, it's just a basic starter kill relay wiringFirst Class MECP Installer

what if i hook a led in between the toggle and 86, what resistor would i need if my relay is 400 ohm across the coil?

You would not have enough current to drive the LED. Run the LED in parallel to the relay CKT and use Ohm's law (R=E/I) to determine the resistor using the values from the LED specs R=(12-LED voltage) / LED fwd current