amps w/out new HU upgrade
hey, the line out inverter only changes the signal from your radio to a low power signal for your amp, it has nothing to do with power or remote turn on wires. you'll need one for each par of speakers. (they usually only convert one set of speakers, sometimes two sets..and they cost 20$ each usually at radio shack or car audio stores, or electronics stores)
Make sure your amp doesn't already have the inverter built in, it may have whats called "high level line inputs" which is the same as an external line out converter
I have a 99 explorer sport, and i did the same thing you are trying to do. What i did though was bought a stereo wiring kit from wal-mart (returned it for my $7 when i was done)to use as a guide to figure out what wires were what behind the stereo, when you match them up, you will find that eight of them are for the speakers(positive and neg for 4 speakers) and three are power, the red will be constant power which you won't use for your amp, the black will be ground which you will not use for your amp either, and one will be yellow, this is a remote power wire and should be spliced into and run to your amp to turn it on and off with the ignition.
if you have any questions, you can e-mail me, also, i have pics of my amp install on my explorer sport 2dr if anyone is interesting, it is in a cool location and may provide some inspiration for you other people, but i can't post the pics since i don't have a web host.
later