keep in mind if you get the MS-8, which you should, you can run the rear and center speakers off of it
and in that case you don't need the OEM HIFI amp anymore
so you can take that out (keep it for resale of car) and you can install the MS-8 in the same location
and if you are only powering 3 speakers using the built in amp, you can even use the OEM power and ground cables from the HIFI amp to power it.
maximum power draw on the ms-8 is 16A, per manual, that's assuming you are powering all 8 channels.
you will be drawing at best about 6-8 amps
which the OEM wire can easily handle
so right there you have power, ground, and remote all conveniently placed for you
so you could even do this without taking apart any trim etc
Are you using Technics harness?