1975 Plymouth duster brake relay question?

As others have commented, that is a mess of wiring. Hopefully you carry a fire extinguisher in your car? Disabling that seat belt interlock is a start but do consider buying a new harness soon. What a dogs breakfast that looks like!

If your Duster still has the fender tag, it will tell you the scheduled production date of it. The fender tag is a ~2x3” piece of metal screwed to the top of the inner fender past the windshield washer reservoir in your photo towards the front of the car. It’s a 3 digit (maybe starts with an A with two numbers or just three digits) to the right of the code that has “000”. Given your 75 has the seatbelt interlock/wiring harness, it would have been built in August (8) September (9) or October (A) 1974.