There is no need to go under the car or wheel arch at all. The easiest way to run cables in the R32 is down the drivers side from inside the engine bay.
There is a triangular shaped grommet in the inner guard behind the shocktower beside the brake booster.
When you remove the triangular grommet you will see a large round grommet in the firewall
directly behind it. From inside the car this grommet is located high above the accelerater pedal.
Remove the driver's kickpanel for better access.
Poke a screwdriver through the round grommet, from inside the car, into the engine bay far enough so that you can use electrical tape to tie the cable to the tip of the screwdriver.
If the cable is thick a bit of rubber grease will help to ease the screwdriver and cable back through the grommet.
Feed enough cable through to reach the amp and run the cable under the edge of the carpet where the sill meets the floorpan.