All right, where did my original post go? I’ll try to recreate.
The scenario you described indicated that you have the common wire and one of the auxiliary wires reversed. Essentially, you’re going to want to pull out your other switch, but do not remove the wires. Have the power off before you do this. After you do these things, carefully separate the 3 wires that were attached to the other switch, the one you removed. The idea is that none of them should be close to each other or anyplace else they may short out to. Make sure, also, to have the one attached switch placed so that it cannot short out anywhere, either.
Now, turn on the power.
With a voltmeter, check to see which wires are live, that is, which ones have power. The analysis is simple from here, but make sure you note your results on paper. For brevity, I’ll use “Connected” as describing anything to do with the one connected switch, and “Loose” to describe anything to do with the switch you originally disconnected.
If you have two live Connected wires, and one live Loose wire - Main power from the branch circuit is being fed to the connected switch. Now, carefully flip that Connected switch. One of the connected wires you previously identified will go dead, and the other wire will remain live. Additionally, the third wire you originally identified as dead , will now be live. On the Loose end, flipping the Connected switch will cause the one wire that was live to become dead, and one of the other to wires will now become live.
Conclusion: the Connected wire that always remained live regardless of Connected switch position, is the common wire leading to the power source. The Loose wire that never became live is the common wire to the light fixture. If you examine which terminal on the connected switch has the common wire, the same terminal on the Loose switch should also be the common one, unless different brand. Usually, it’s the screw on the side all by itself.
If you have one live Loose wire, and no other live wires – Conclusion: the Loose live wire is the common wire leading back to the power source. From there, you should be able to find the common terminal on the Loose switch, and be back in business.
Now that you know the common wires, you may be able to get your X-10 module working properly. As an FYI, my home has the predecessor to X-10, General Electric’s Remote control, Low-voltage switching system. Built in 1954, my system employs relays, and has lights and outlets controlled by as many as 6 switches located all over the house. The only drawback is that it is not wireless.