Last week I put in an Alpine H/U. My particular model does not have an internal amp, so I used the stock amp by running the preouts to the speaker level outputs of the amp (essentiailly the stock speaker wires on the harness). This also allowed me to continue to use the stock sub. I attempted a lot of tiral and error and will add all that I have learned. Excuse how dirty my car is.
To get to the radio first remove the silver trim panel above the glove box.
Open the glove box.
Pull the panel off from the bottom.
Panel Looks like this. If it doesn't match put it back haha
Next pop the top edge of the trim piece below the steering wheel out and then remove the trim panel with the driver control center buttons. It is on the left of the radio trim piece and right of the steering wheel.
You will have to pull on the tab, shown in the picture below, to remove the center trim panel. It is the only real place to get fingers behind and pull. It is possible to pull some on the bottom, but I was never successful. The trim piece resists strongly, but didn't break. It may need to be worked awhile, but it will come off.
Here is how it looks once the trim and radio are removed. Four screws hold the radio in. The wiring harness adaptor ($5 from a local place I have connections) is plugged in for a test look. I removed all the excess wires I didn't need after this.
I removed the Stock Reciever relocation plug (used for recievers that have driver info built in). Can't think of anything else I removed, but don't remove the power antena/remote line of the harness. They are needed to turn on the factory amp (I made this mistake, but luckily kept the wire).
Here is the factory radio removed.
Now its time to play with the wires.
This is normally straightforward, but I'm going to go over it anyway.
Yellow on the adaptor harness goes to the constant power of the h/u harness
Black on the adpator goes to the groud of the h/u harness.
Blue is a power antenae lead or as used in the Pioneer system a turn on for the factory amp. This will attach to the remote line of the h/u harness. If you need to run any other amps connect their remote lead here as well. Use a twist cap, so remote lines can be added that do not run through the harness. It makes removal and install easier.
There is no turn on/ignition lead to turn the h/u on and off with the harness. These new radios use a fancy bus system to turn on. Another member here has mentioned the only way to utilize this with an aftermarket radio is to buy a harness costing over $100. I am bypassing this system for that reason and some would consider it "ghetto", but it works. To turn the radio on and off with the car run a wire from the ignition fuse in the fuse box to the ignition lead in the h/u harness. Use a twist cap here as well.
I tried searching for a switched wire on the inside of the car, but gave up after a half hour or so and hooked it to the fuse. I plan on going back and searching some more. This would give a cleaner install.