When I put in my hands free cradle I wired to the fuse block. Get a multi-meter.
(forgive me for going detailed if not needed) Set on VDC. Put back lead on exposed body metal or batt ground. Put the red probe on fuses or put small nicks in wires that you can tape up and look for 12VDC with the key in "on" and "acc". Turn it "off" and look for it to go to "0" and you found your switched 12VDC.--BUT what does it use? I think some are only for a 5v hook up from the headunit. If your going to tap a fuse I recommend not taping airbag/PCM/inj or any fuses rated 5A or less.
If you have the BOSE system, you can find it in the trunk, driver'* side of the back wall (behind the back seat).
Pull the big plastic wall cover off (on my 2000, you have to unsnap some worthless plastic brackets that are under the rear deck speakers), and you will see the BOSE amp and assorted speaker wires feeding it. The PINK wire is the remote on wire. You can tap into it for remote amp switching.