I have a 1997 Eclipse GSX with an Infiniti Sound system (not sure if it's stock or manufacture add on)

I'm trying to install an alpine deck and I have a wiring diagram from modifiedlife.com and it's not 100% accurate

Questions:

1.Does the Infiniti system use an amplifier? I believe there is one under my passenger seat but I never got a chance to remove the seat and look.

2.If it does have an amp; Do I have to connect the Alpine Deck to the Amp in order to have power routing to it?

3.Has anyone removed a Infiniti Radio/CD/Tape deck to install an aftermarket head unit? If so, common problems/fixes?

Anyhelp would be appreciated, thanks!