If you have limited experience with automotive electronics I would suggest you get this done by a professional.
SKILL LEVEL: 6.5 out of 10 (my opinion)

Before you start, make sure you have everything you need because you don't want to start, and then realize you're missing something and have to undo everything you just worked so hard on.

Next to the basic wiring and soldering tools you will need:

-head unit
- wiring harness adapter
- antenna adapter
- dremel (best and easiest option)/hack saw/jig saw
- 600-800grit sandpaper

First thing you want to do when working with ANYTHING electrical is disconnect your NEGATIVE battery terminal.

To begin, tilt your steering wheel all they way down. This will give you more room to work with when removing the bezel. Then take the two screws out of the upper part of the gauge cluster holding the plastic to the dash.

I put all of the bolts and screws in my cup holders so they wouldn't get lost.

Pull the bezel off only enough to undo the clips holding it to the dash.


Unclip the 4WD harness from the back of the switches.



To give yourself more room you can either uncoil the harnesses from the headlight/wiper/hatch switches or completely remove the switch master from the bezel. I didn't do either. I pull the bezel out far enough to where I could just flip it up and over the steering wheel and lean it against the drivers side a-pillar. You may have to put your shifter done into 1st to get enough clearance to get the bezel off.

You now have access to everything you will need to remove as far as a head unit installation goes.

Remove the bolts holding HVAC controls, head unit and lower pocket to the dashboard. They are all the same so you do not have to worry about putting them back in the same holes.
Pull the HVAC controls out and let them hang out of the way.


Pull the head unit out carefully remembering the antenna and wiring harness are still attached to it. Once you have it out far enough. Unclip the wiring harness and antenna from the back of the headunit and place it someplace it won't get damaged.
Remove the pocket and put it out of the way for later.
Your dash should look like this at this point:


NOW THE FUN BEGINS: CUTTING

Remove your double din head unit and single din dash kit from it's packaging.
Using the BACK of the head unit, hold it centered against the front of the dash kit and mark how much you need to trim off the bottom.

Using whichever tool you feel is best to cut a clean straight line (I used a hack saw), cut the dash kit to the markings you made previously. Sand down all of you cuts so there are no sharp edges that can cut yourself or any wires.



Test fit the dash kit to the head unit and make any necessary adjustments if needed.

Find a way to secure it the the head unit. Mine fit almost perfectly to the screw holes in the side of my unit but you may not be so lucky. Some creativity may be needed depending on what dash kit you get.


*I dont have any pictures of the next dew steps. If help is needed, please ask.*

Temporarily attach the head unit (with dash kit attached) to the dash board location being sure to support it from the bottom so the plastic does not break.

9 time-out of 10, a double din unit will be too deep to fit in the PRE 98 blazers (mine is a 97 so I'm not sure for 98+). This mean you will have to cut out the plastic in the back of the cavity. THIS IS VERY IMPORTANT!!! You have to be careful when cutting because the dash harness wires are directly behind the area you are about to cut. The easiest way I found was to mark where I need to cut and then use a box cutter to "score" the cut lines. One I went over the cut lines with a box cutter a few times I then took a pair of needle nose pliers and carefully broke of the section the needed to be removed.
This will allow your head unit to slide into the dash far enough to sit flush when everything is put back together.
After you have "fit" your head unit temporarily install it once again and slide the pocket in and mark what needs to be cut if any for it to fit properly. I needed to cut the side tabs on mine but you may not need to.
When you are happy with the fitment of both your head unit and storage pocket, you are ready to move on to wiring.

***I am not going to go into to much detail with this as there are many different techniques. I suggest reading the instruction that came with your specific head unit and to solder and shrink wrap all of your connections.***
Once you have wired your radio harness adapter to your head unit harness, connect your negative battery terminal and test you connection to see if the head unit it working properly. If it is, you can move on.
First install the lower pocket.
Second install your head unit being careful not to pinch any wires when pushing it into the dash cavity. Don't forget to attach the antenna adapter and cable (pretty self explanatory). Tighten the scews the hold the lower pocket and dash kit the the dash.

Then re-install the HVAC controls and tighten the securing screws.

ALMOST DONE
Time to reinstall the main dash bezel

Line it back up with the clip hole but DO NOT clip it in just yet. You need to re attach the harnesses to the switches you removed earlier.
Once your switches are connected, Carefully go around the dash bezel and push the mounting clips into their locations. Once that is done don't forget the two screws in the top of the gauge cluster.

Sit back and enjoy your new head unit...
**Don't be alarmed with how far mine sticks out from the dash. It a cheap $150 head unit. You get what you pay for. A decent touchscreen double din will run you about $800-$1000. When you spend that much on a unit. It will more than likely sit flush with the dash kit. I didn't want anything crazy, just an upgrade from stock with the ability to use a scan disk and DVD.***



If you have any questions, don't hesitate to PM me.

Good luck!

 

Pictures added.