I didn't have a '67 FSM, so I'm assuming that a '66 (downloaded one) is wired about the same and the two wires going to the switch are the same color.
So... That means a little detective work has to be done on the car that I can't do from here.
There are 2 wires going to the switch. One is for the lights and one is the power for those lights. You will need to figure out which one is the power using a test light or a meter. This probably will only be powered when the ignition is on.
When you find that, label that wire "Feed/Switch 1" and the other wire "lights/switch 2"
When you have the relay, you'll see the contacts are labeled. Here's a crappy pic.
This is how it will be hooked up. The "lights/switch2" wire is cut and the output to the lights connects to #87 and the wire from the switch connects to #86. The "feed/switch1" wire is connected to terminal #30 and then on to the switch.
#85 connects to a good ground.
Not shown but you should add an inline fuse from terminal #30 to the lights. I'm not sure of how many bulbs are at the back of an Imperial, but a 20 amp fuse will keep everything from burning down.
You can use female spade connectors and then some heat shrink tubing to insulate it or order some of these:
https://smile.amazon.com/ONLINE-LED...1&keywords=relay+socket&qid=1615903913&sr=8-3 or these
https://smile.amazon.com/Relay-Conn...1&keywords=relay+socket&qid=1615903987&sr=8-4 for a neater install.
Relay 101 edumacation...
#30 is the terminal that you bring your main power into.
#85 and #86 are the terminals you use to make the relay work. These can be reversed, but one will go to ground and the other to your switch.
#87A is connected to power when the relay is not energized
#87 is connected to power when the relay is energized.