He'll need a circuit breaker or fuse within 18" of the battery, and a heavy enough gauge of wire to go to the amp.Plan on using at least 4 gauge so that, if other amps are to be added later, a distrubution box in the trunk can split that into multiple fused leads to more amps.
The signal can be taken from two sources. The best is from a pre-amp level sighnal (from the RCA-out jacks in the backof the source unit) . Some source units have subwoofer outputs that are already low-pass filtered so that only the lower frequencies are amplified for the sub speakers. Although, since the right X-ver frequency and slope is highly specific to each individual car's acoustics and speakers, these source unit sub outputs are rarely useful. You'll need an electronic crossover which will take a full-range pre-amp signal and separate it into high-pass, low-pass or band-pass frequency bands. It takes a long proces of trial and error (and listening) to get the right cut-off frequency, the slope (a curve of how sharply the X-ver filters out higher frequencies-- expressed in db/octave) and phase.
If you don't have pre-amp level outputs. Use can use the speaker-outputs, but they must be converted to a low-level signal first (some amps and crossovers can do this)
JL Audio just came out with a device that takes the speaker output from a factory source, converts them to a low-level signal, converts that to a digital signal which is analyzed and EQ'ed automatically from a microphone placed in the car to make up for anomalies in the acoustics in the car, and then the device outputs several RCA outputs for various amplifiers. This is a great option if you don't want to change the stock head unit.