If you have a 3-stalk cluster, you'll find a PN # on it. I suspect it will start with 993 (if the connectors are incompatible). Also, you'll see that the OBC stalk will detach from the rest of the unit - it just screws off.
Were it me, I'd remove the 3rd stalk, implant it on the OEM cluster on your car and do one of 2 things. I'd either find the compatible Molex connector for your car and transplant it to your new stalk, or get a generic Male and Female 4 wire (? IIRC) connectors and splice them in place of the stock Molex connectors. Pretty easy to do. Also, you'll want to cut the rubber cover on the escutcheon to accept the additional stalk with a razor blade or such.
Here's the write-up:
http://www.whiteson.org/boxster/mods/obc/obc.stalk.pdf
