6x9 speakers are great for adding more volume for the rear stage of the car and more bass because they have lots of cone area and are cable of handing lots of power. Good quality pair of 6x9 speakers from a brands like Alpine and JBL are around 100 rms, cheap Walmart ones are around 50 watts rms. They can be fitted into the rear of your vehicle, by installing both speakers into the stock parcel shelf, you can have an special stealth shelf made or install them into premade 6x9 enclosures and place the boxes on the rear seats.
In car audio, there are two ways of powering them, directly from the head unit or from an additional amp. If you want an simple install and don't want the hassle to install the wiring for a car amp and rip apart the vehicles interior, then wiring them up to your deck is the best way, however if you want really loud volume, adding a car amp to power the set of 6x9 speakers is a better option, as it can provide an louder output signal to drive them. The main difference is wiring them to the stereo is easier and you don't have to spend more money, as you don't have to buy an car amp and an wiring kit, but the speakers will not be operated to there full volume output potential, as most head units have mosfet output of around 25 watts rms per each of the 4 channels, which is nothing really, if you are wanting crazy loud music. Plus when your increase the volume level to its maximum on the radio, running them off the stereo, you will hear distortion, making the sound quality sound crap because head units can't produce clean power at nearly full volume, as they have crappy built in amps. So adding an amp to connect to the 6x9 speakers is the best way to go.
I would recommend you to use an car amplifier that matches the rms of the 6x9 speakers you want to hook up. This is so you don't under power them or overpower the voice coils, as it can cause the speakers to get damaged either way. For this car audio installation, i would suggest buying an 2 channel amp, but if you have an 4 channel amp and already installed the front speakers to it, you can use the other 2 remaining channels for the 6x9 speakers. Please don't buy an car amp that is a mono block, that is designed for an subwoofer.
1) Once you have installed the car amplifier into your vehicle, you will need to buy the correct length high quality speaker wires, that is about 12 gauge. To find out how long the wires have to be you when you make the purchase, measure how long the wires would be, from the amp output terminals to the terminals placed on both speakers, underneath the basket.
2) Once you have the speaker wires, remove the 6x9 speakers from there location and access the terminals, underneath the speaker. There will be two spade terminals, one will be positive (+) and the other will be negative (-). Using a wire stripper strip of 1cm off plastic coating from all the ends of both speaker wires.
3) Once you have done that, you are now ready to make the connections onto the speakers. So using an soldering iron, solder the wires accordingly to the terminals and the colours on the wires. The speaker wire with the black stripe down it, has to be soldered onto the positive spade terminals on both speakers and plain speaker wire, with no stripes or markings needs to be soldered onto the negative spade terminals on both speakers.
4) Access your car amp, look for the speaker terminals, they will be labeled, showing you an small, simple wiring diagram on how to wire speakers to it. This installation requires 2 channels, left and right sides. So get an screwdriver and unscrew all four terminal screws.
5) Once the screws are loose, you want find out which side is left and right. So first find the left side channel, insert the positive speaker wire braid from the left speaker into the terminal marked as '+', by pushing it in, underneath the screw and repeat this step, for the right channel, using the positive speaker wire from the right speaker. Then insert the remaining speaker wires underneath the screws, in the terminals marked as the (-), the left negative speaker goes into the left negative speaker terminal and the right negative speaker wire goes into the right negative speaker terminal.
6) Check all the speaker wires are inserted firmly into each terminals and using your screwdriver, screw the screw in each four terminals tight, to prevent the wires from coming out, which can cause crackling later on.
7) Power on the head unit and play a music track to test if the speakers are working, increase the volume a little, if you hear sound coming from the 6x9 speakers, the install went successful. If you don't hear sound and the 6x9 speakers are not working, then check both ends of the RCA Cable are plugged in properly into the amp and rear preamp outputs on the deck. Also check if the speakers wires, or remote turn on wire are not loose. Now all you need to do is set the gain control on the amp and enable the high pass filer hpf, setting it around 60Hz, if it has an adjustable screw.