installing stereo in 03 envoy

It has a class 2 databus system as I described on that Avalanche you did. They are almost identical setups. As I suggested before I would replace the factory radio, the sound quality it puts out is not that great. Now your car dose not have a remote/ switched 12volt, so you will need to tap into the pink wire in the ignition harness for your remote wire to the amps. Another problem caused by this is your amps will be on as long as the ignition is on. If you have the gains turned up too high it will have a fuzzy sound when your radio is off but the amps are on. I would suggest you install an in line toggle switch to turn the amps off to avoid any unwanted noise. To hook up your high to low converter you must tap in after the factory amp. You will also need 3 sets of rca outputs, your converter will only give you 2. Now if your amp has an rca output/ pass through then you can use that to connect to your sub amp. If your amp dose not has that, you will need to put an rca Y adapter http://www.knukonceptz.com/detail.asp?product_id=KLS1F2M on your rears. From that y one set of rca's will go to your 4 channel amp for your rear mids and highs, and your other set will go to your 2 channel sub amp. Also if you do use the factory head unit I would suggest you use a 2 / 3way electronic X over to adjust highs mids and lows for better sound.