Hey I was just wondering if anybody could tell what to get an what to do. I'm wanting to install my amp an sub in my truck but keep my stock radio which is the navi.. I'm just woudering if you guys could help me with this...

 

The simplest way is using just an amp with speaker level inputs and tapping in to one of your rear speakers. A better way is with a summing Line level output converter (LOC) like the AudioControl LC2i to create your sub channel by summing the stereo output instead of just grabbing it from one speaker. In most cases you would not notice as sub bass tends to be mono but you will get better results by summing a stereo pair into a mono signal for your sub amp. This can be done by tapping in to both rear speakers without having to "open up" the dash.

 

i just installed 3 amps, 4 speakers, and 2 subs using my stock headunit....

if you want to power your speakers....i recommend getting a junkyard HU pigtail and then a wiring kit so you dont have to hack up your factory wiring. although i not necessarily needed.its like over 100' of wire if you mount the amp behind the rear seats.
you just take the wire that comes out of HU-> run to amps using line level inputs or LOC->then back to factory harness. its that easy. one thing i did notice is more white noise/static if no sound is playing when you run more juice.

if you just want to add a sub you can find the rear speaker wires under the driver door sill. this way you dont have to take HU out. just use those t-connectors and you dont have to hack anything. running line level into amp is the easiest method.