I just replaced the head unit in my 99 3000 SCR. The wires come from just below the Master panel to the left.
I also added 2 amps, 2 subs and 4 more speakers, remote on transom, remote CeNet and remote MP3, so I used a cordless drill with a 1 1/2" paddle bit to drill a whole for new wires. There is a plywood wall there that prevents you from not drilling.
You will need a small drill like the new Makita 18v to fit in this area.
On the 3000 there is a precut access panel below the sink to access the port full tank sensor.
You can use a feeder (coat hanger, fish tape) to run more or a new power wire to you Stereo head unit.
This area is really tight for visual while drilling but it is doable.
Hope this Helps!
We also made out New Years Eve to enjoy the 75* and do a little dog training.