When I wired my 110 I ran all the wires down to the fuse box (there is a channel behind the dash)
from there its your call. I had no wires at all, so some speaker wires were tucked in under the dash to the front doors.
I rigged some and strung them underneath like a Christmas tree lights....
Riggy, but its just speaker wire....worked fine. Same with sub wires, under the body
If you are going to tap into the harness that is under the tranny cover you may can run the wires over the tunnel but under the mat?? You can use your old wiring harness or tap in direct.
I tried to use the old harness, but the connections were corroded. So I had a stereo store fix it when the made a new sub box and added an amp.