I've fitted many head units and amps in my cars, mostly Kenwood and Alpine, quality goods. And the hole in the passenger footwell in civics makes wiring an amp a doddle. Not so easy is feeding the amp cable under the plastic sill covers and into the boot remaining hidden. Some people suggested I feed the cables under the car...oh how i lolled. That's just a blown battery waiting to happen.
Thankfully those days are long gone, now I simply upgrade the head unit and i'm done. Once you start fettling with amps etc, it goes on forever and ever.
Nope, call me old fashioned, but a HU upgrade is enough for me nowadays.
