I got to the siren and discovered it was disconnected but externally looked new. I plugged it in overnight to see if the batteries would recharge. It appears they did not..

I checked the wiring going to the siren. R/Y was 12.28v, Y/R was 11.8v and Brn was apparently ground or at least it completed the circuit to the other two. I understand that the 3.6v batteries are being charged via one or both powered wires but am left wondering how this kind of circuity can produce the signal to activate the siren. Perhaps a voltage change triggers the IC. It's my only guess. I wanted to make certain that I'm not replacing a perfectly good / expensive device. I checked across the receptacle and could only get .45v or nothing at all. Continuity was was 0 ohm with any combination of tests.

Since I got so little voltage between the brown and red/yellow wire I figured the batteries were not charging and cut the siren apart like others have done. The circuit board was blackened nearly throughout - both sides. Sooo, I guess I'll plunk down the money for another siren and hope it doesn't destruct for some unimagined reason. I know the resident electronics expert is away but perhaps I'll wait an hour or so just in case he is monitoring the forum remotely..