Cross posted from your other question:
Hello, it depends on the year of W123 you have.
The very early models with the "big" hazard flasher switch that you push in to turn on the relay is part of the switch. You simply replace the switch as one unit along with the relay.
If you have a later model W123 with the smaller hazard switch that is a toggle style switch instead of a push button switch then the relay is underneath the wood trim that surrounds the shifter. Search around online on how to remove this wood trim - generally it's pretty simple. You remove the ash tray and the bracket for the ash tray and then you can push the wood trim towards the back of the car. It will move backwards at which point you will want to carefully pull up on the back of the trim panel. Be careful not to damage the pin at the back of the trim panel - you need to pull it back far enough to clear the opening that it goes in to! Once the pin is clear the trim should just lift up over the shifter. You will see the relay hiding underneath - it should be the only relay there.