Looking at a couple different options for my 04 Discovery 2 with the Non-nav alpine unit with tape deck and CD changer.

1. Use an android tablet and a bluetooth solution like some of the Parrot devices attached to an aux line

2. An aftermarket stereo that does not look like some cheap crap that you get at walmart.

3. Aux input to keep factory stereo using CD changer.

My problem with #3 is that if I have to buy that $75 part I would MUCH rather have a better head unit for $100 with the ability to change the sound more than just BASS and Treble..If I could do #3 without silly $75-100 blitz or grom part I would just do that first and see if I like it(no need to keep changer).

Also for aftermarket do we need 2 sets of pre-amp outputs since we also have a sub?

 

i have seen the grom/metra style on ebay anywhere from $15-50, yes you lose the cd but who uses a cd anymore

 

I suggest getting a good after market head unit. The stock one is outdated anyway.

 

You do have some options, some more expensive than others.
1st is keeping your stock unit an adding the Grom unit and is expensive if you want bluetooth and phone hookup
Compare GROM Car Audio Integration Kits
2nd replacing the head unit, just note the opening is 1.5 din and the hole to install the aftermarket head unit is 1 din, therefore you would have to purchase the metra piece that clips in to fill the gap below the aftermarket head unit.
So the aftermarket head unit would consist of head unit, metra harness adapter, PAC-SWI steering wheel control module, and diversity antenna adapter if you want to use both antennas that are in the back windows.
The factory amp is a 5 cannel amp and takes the sub signal from the 4 signals coming from whatever head unit you use, either stock or aftermarket(so no you do not need any extra outputs from head unit) Your aftermarket head unit should have sub output and this would only be used if you are adding a sub and mono block amp.
On the back of your factory head unit, there are 3 colored mini-iso connectors, yellow, green and blue. The blue one goes to the cd changer under the passenger seat, this is where the GROM unit attaches, I found this on amazon and it should work, but can't guarantee it. It is cheap and worth try,
Amazon.com: RSD 1.5M AUX Audio Cable Porsche Becker CDR 220 and CR 220 with Radio Removal Keys/Tools: Automotive

 

I had the same decisions to make on mine, and really wanted to bring the system up to date.
After much research, I decided to go the aftermarket stereo route. I wanted it all: DVD, Navigation, HD Radio, SiriusXM, SDCard, USB x2(1 for phone, 1 for external hard drive enclosure)
I am using all the factory wiring and amp and speakers, and added a 10" sub and mono bloc amp(removed the cd changer and installed the amp in its place.
You use the pre amp outputs on the head unit and not the speaker outputs(the metra harness has rca connectors making that part a breeze.
There are two antenna leads (1 large, 1 small) and there is a diversified antenna adapter for 20.00 that connects them both to the head unit, or you can just connect the large one(small one is fm only, large one is am/fm)both of them have a signal booster hooked up and they are turned on the same way the amp is turned on, via amp/antenna lead from the head unit(usually blue with white stripe).
I took my center dash fascia out and cut a hole large enough for a 2-din head unit, lowered the heater controls to make room and added headrest monitors for kids to watch the videos that are stored on the hard drive.



If you need help with anything, let me know.

 

I've detailed it elsewhere, but my far cheaper and satisfying solution was to just splice into the CD changer under the passenger seat and add some 1/8th plugs into the compartment between the seats (I also added a power outlet) . I have a bluetooth unit in there and just run off my iPhone.
Works great.
Neat and easy and cost about $20.

 

Do you have details on how to do this?