Test it before splicing! Grab a multi-meter, test it with the radio off (antenna down).... Should rest at 0V. Turn the radio on, if it switches to 12v its your acc, ign or antenna....try turning the radio back off and jumping an alternative 12v source (i.e. white wire in ignition harness) to it w/ the radio off. If the antenna goes up, your golden.

If not, do it the easy way... grab it directly from the back of the radio.