I had one but it has been a long long time ago. Don't remember on that one but I don't remember a TT switch on the starboard side of the lower cowling like later models......which would allow you to see if that switch did or didn't make the down trim work and then know if your remote switch or connection to it is the smoking gun.
If your color code is blue sky up, green grass down, and you put your meter on the same terminal on both relays and executing the correct trim switch position, one gave you voltage to the/that relay and the other (green) didn't then I would suspect the switch in the control. Quick check is to swap blue and green wires at the relays. If the UP switch position on the control now makes the trim go down, you have a bad switch in the control.
If that doesn't work then do you have 2 trim relays? If so, and they are identical, put the wiring back where it was initially and swap the relays. If you get down trim then yes the original down relay is malfunctioning. Just for grins, unplug both and reinsert them, basically burnishing (cleaning) the relay contacts.....if plug in type. That may be your problem. If lug and screw type I doubt it but the terminal crimp at the screw could be corroded and not passing current......or allowing you to measure voltage at the lug or screw/nut on the down relay.