My GTsT had a factory Nissan "active speaker" stereo system. This sends a "quiet" signal from the head unit which is amplified by three auxilary amps arround the car (the amps all need power and a signal to switch on and off which is supplied by some extra wiring)

This won't work properly with anything but a Nissan "Active" head unit and mine had a broken tape deck! So here's what I did;

I decided to fit new front 4x6 speakers as the old ones were knackered, but keep the OK sounding 6x9 rears. As far as possible I wanted to keep the original wiring.

Replacing the front speakers takes the two "active" amps built into the front speakers out of the equation (here's some help on doing this from Brendon Moses), you just need to safely insulate the extra "power/switch on" wires. I still had to bypass the single active amp used for both the rear speakers wihch is under the parcel shelf between the HICCAS brains.

I looked up the wire colour codes at SDU and ran new wires from the connector straight to the speakers. This worked OK but it was a bit  messy. Then I realised that you can easily bypass the old amp whilst still using its neat wiring loom... Unplug the connector and cut it off and join the wires taking signal from the head to the amp and from the amp to the speaker as shown below. I used a chocolate block. The pairs of -/+ wires were taped together in the loom already which makes it a bit easier. The three centre wires need to be safely insulated as the carry power to the amp.

Now go and have some osteopathy because leaning into the boot and looking up at the underside of the parcel shelf is a serious contortionist job.

Alternatively

If you want to keep all the speakers and you don't want to cut any wires you can try to send a 12v Switch ON signal to the correct wire of each amp, the amps should in theory stay on all the time. The downside of this is that the amps will be boosting a signal that is already full strength which may cause them and/or the speaker to distort at higher volumes. They may also put some extra drain on the battery.