The only wire left to do before any testing was the remote power lead. This wire turns on the amplifier when it receives a 12V turn-on voltage. This is usually a much thinner wire than the power and ground connections for the amp, since it draws very little current. It's typically about 18 ga.
I have the UP3 stereo (CD in dash) with the rear-window antenna. The presence of the rear-window antenna makes it very easy to get a turn-on voltage for this lead. The wire to tap into is the yellow lead that goes to the antenna amplifier. It turns out that the antenna amp needs the same turn-on voltage to do it's job. The bonus is that this lead is also hot when using the CD. (I cannot speak for other stereos, or for cars with the mast antenna. If you don't have the rear-window antenna, you may have to get a signal from the instrument panel fuse block)
