Speaking of being warned, I also posted over on MVS and a guy there warned me to not wait until the battery leaks acid all over the circuit board. I did a little internet searching and found the OE power source is a stack of (3) x 2.4V, 240 mAh NiMH large button batteries (approx 1" dia x 3/8" deep) = 7.2V total. Apparently that type of battery is difficult to find stateside.

So I have a "plan". I went by Batteries+ and I'm having the guy package (6) AA rechargeable NiCads in series (= 7.2v) with pig tail leads. They spot weld the bridges between the batteries rather than solder and then shrink wrap it all together. He's charging me $25 which includes batteries and packaging. I'm guessing the new battery pack is unlikely to fit inside the siren box, so the "plan" is to mount it remotely inside the ECM/TCM box with a disconnect there to facilitate subsequent replacements. It will only be about a 12" run down to the siren box. I'm thinking that should be a fairly good environment with the cooling fan. According to him the NiCads will discharge faster than NiHydrides if the alarm goes off, but overall they'll last longer before needing replacement. It will be interesting.