How to “listen” to existing circuit
A relay is almost certainly NOT the right tool for the job.

The first step in wiretapping an interface in order reverse-engineer it is to determine the nature of the signals you want to look at. For voltage signals, you need a high-impedance buffer amplifier (like the input of an oscilloscope) that will cause minimal disruption to the existing circuit. For current signals, you  need a low-impedance shunt or other measurement technology (e.g., Hall-effect), like the input to a milliammeter.

Other kinds of signals (such as capacitive, inductive or resistive sensors) might require some special ad-hoc techniques.