Audio
There's no doubt that having a decent stereo is important, and quite honestly the one fitted in the factory isn't the best, and the speakers are basically pants. Most vehicles these days use ISO standard connectors allowing a range of head units to be connected, older vehicles will need custom wiring.
Head Units
If I could give you one piece of advice when choosing a head unit, it would be to avoid cheap chinese, buy a reputable brand like Sony, Pioneer, etc and you won't regret it. You should also be aware that some vehicles do not support double din fully, instead having plastic in the way, which you can of course cut away or get a head unit that has a double din face but the actual unit is single DIN - of which there are many.
Here's a few suggestions:
Speakers
Good Speakers are vital for an acceptable experience, and the ones factory fitted are really poor (You'll see when you swap them out). I can seriously recommend Vibe Audio, a brittish company providing high quality speakers at a sensible price. Vibe also do a suitcase sub that fits nicely under the front seats to provide some bass to compliment their mid range door speakers.
4”(100mm) 2-way high efficiency coaxial speaker 120 Watt, Peak Power output – 40 Watts, RMS Frequency Response: 100HZ – 20KHZ, Sensitivity 83dB, Mounting Diameter 3.7” (95MM), Mounting Depth 1.7” (44MM), Impedance : 4 OHMS, Included Components: speakers, manual | Buy | |
VIBE SLICK 10 Inch 3000W SPL 1500W Max Car Audio Bass, 500 Watts RMS, 1500 Watts Peak, 3000 Watts SPL (BURP), Dual 2 Ohms, 2 Inch OFC Voice Coil, Included components: subwoofer, manual | Buy | |
Excellent Pair of Mid Range 6“ Door Speakers, Non pressed paper cone, Dual roll cloth surround for extreme midrange performance, 25mm voice coil, Polycotton spiders, Spade terminal for easy connection | Buy | |
BlackDeath 6 Inch Pro Audio Midrange, High Output 6″ Pro Audio, Alloy Gasket included, Speaker Size : 6″ (170mm), RMS Power : 175 watts | Buy | |
An Excellent Under Seat Suitcase Bass, RMS power : 300 Watts, Max power : 900 Watts, Impedance : 4Ω, Type : Slim fit bass enclosure, Configuration : 4th order bass reflex, Active amplifier : POWERBOX bass 1 (included), Dimensions : (H x W x D) 3” x 10.6” x 15” (75mm x 270mm x 380mm), Weight : 4.66 kg | Buy |