3 Terminal Relay Question

Have a very old truck and there's a 3 terminal relay that clicks whenever brake pedal is depressed.  It's old and rusty and sometimes stops working.  Was looking to replace it with a 5  or 4 terminal relay but wasn't sure how it was working off 3 terminals.  Do you think the relay itself bolted to trucks body is acting as 4th terminal?  If not I guess it's no big deal for me to just rewire a new setup from brake switch to a new 5 terminal relay.  I'd just like to know how that 3 terminal relay works.. Thanks for help

I connected the mutlimeter to each of the 3 wires and tested current and voltage while pressing the brake pedal and releasing but no changes on any of the wires. 1 wire is constant 12 volt it's a thicker wire and there's another green wire that's thick.. the 3rd wire is a thinner wire... guessing that coming from the brake switch...but when i checked it with a meter it had no voltage when i pressed and released pedal.. when i connect those 3 wires back to that relay and press the brake pedal it clicks but i don't detect a change in any of the wires.. how's it working?   Any ideas?

Got pics?  Most likely the + side of the coil and the + input to the contacts is the same terminal.  The other thinner wire might possibly be the - side of the coil.  Or, it's reversed: the contacts switch ground and the + side of the coil is the thin wire.

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but the thicker + wire is constantly powered. If they're the same terminal wouldn't that mean the relay is always switched?

the thick wire showing 12v+ is a constant that supplies power and acts as a 12v+ to energize the ciol of the relay. the small one is a neg trigger and the other large one should show 12v+ supplied from the 12v+ constant when the neg trigger is supplied.