9 Tips for Installing car audio
1. Don't use cheap speaker wire. That really thin cheap wire really drops off the sound quality at the speaker. It doesn't cost much to get good quality fine stranded dual core speaker cable.
2. Solder your connections. Crimping connectors can lead to poor connections and higher resistance joins therefore reduces the signal getting through to your speakers.
3. Planning. Don't cut anything until you have checked it thoroughly. Both sides, all sides, in and out. It is better to use a board on a back deck than to cut open the deck to make bigger holes. Many cars have the small 3x4 inch holes than won't take 6 inch round or 6x9 speakers. Better to make up a board to raise the speakers (an improve resonance) than to cut out holes. In many cars it is illegal to cut too much of the metal away in the rear deck (supposedly compromises body impact integrity...).
4. Testing! Always test everything before you put trims back on and screw the head unit into the dash. It is easier to fix problems with everthing still out.
5. Instead of using the earth in the wiring loom, find a screw that goes into the metal of the body to earth your amps and head units. Poor earthing causes untold problems from scrtchy sounds, signal clipping, tapes playing all wonky, reduced radio reception, poor tolerance to bass boosting. Also, unbalanced earthing between amps and head units cause nasty nasty static or humms.
6. Make sure you KNOW what that wire you get constant/accessory power from is for! If in doubt run new wires directly from the fuse box (yellow for accessory and red for constant). Be careful and make sure there is a fuse in the circuit somewhere.
7. Make sure your speakers and amps are balanced. When buying speakers some of the better quality ones are rate with RMS (Root Mean Squared) others are rated with Max (or PMP - Peak Music Pressure). As a rule of Thumb 50W RMS would perform as well as 80-90W Max. In fact, Many speakers that say they can handle a millisecond burst of power to 200W (for example) are really only about 80 watts RMS on a good day. Many car amps are the same. that 4x150W amp is probably only putting our 4x60W RMS.
On that note, If you have 4 quality 50W RMS speakers they can produce more sound than your ears can hear. If you run them with a 4x200W (Max)cheapie AMP they should rock!
8. RCA from Head unit to AMP is ALWAYS to be preferred.
9. OK, in general the sound is only as good as the weakest component. I find that good wire, connections, power make a huge difference. Average and even cheapie amps do fine (if they are big enough). Speakers make a big difference especially with bass response. Most head units (except for crap cheap ones) produce good quality CD audio these days so are not as important.
Hope this helps,
(Used to own a car stereo and alarm installation business)