My '06 DB9 came without a battery conditioner. While the information is out there if you look hard enough, I thought I'd post my solution for any others who might need a replacement:
1) I chose the CTEK 7002 as the replacement. It's more than just a simple float charger, but it's far less expensive than the Aston Martin-branded replacement and includes the best and most-modern battery maintenance and charging options, along with a 12-v power supply which can be handy when disconnecting the battery.
2) My car has the 3-pin XLR-type connector in the boot rather than the cigar-lighter socket. I bought a Neutrik XLR plug from an electronics supply shop and soldered it to a set of auxiliary leads that came with the CTEK. (The CTEK 7002 doesn't come with a cigar-lighter plug either for newer AMs, but they're available at any Radio Shack).
3) In my car's boot, the charging socket has the three pin receptacles in an inverted "v" configuration like this: ^ . Looking at the socket from the boot side (outside the car), the LEFT LOWER pin socket gets the red wire, the CENTER (apex) pin socket is unused, and the RIGHT LOWER pin socket takes the black wire.
Everything works great. The charger took the battery through its testing and maintenance cycle, and it's now relaxing in a pulse-charge state waiting for me to return from an out-of-town trip.