UKSaabs

Apologies if this is an old question.

Easy enough to upgrade a 2004 93 stereo?

The short answer - no.
The long answer is very long and depends upon what your starting point is (4- or 7-speaker system) and whether or not you intend to upgrade the features alone or both the features and the quality/quantity of the output.
There is no universal "right" answer AFAIK.

_________________
I had a meteoric career.
Meteors plummet in flames.
Career is what happens when you lose control.

If you just want features such as Bluetooth streaming, just buy an adaptor that plugs into the aux input in EHU (behind the climate controls). Activate aux via tech2 if required.

If you want to install something that does navigation, hands-free, reverse cam, steering controls, and everything else possible, then you have some work to do. The easiest way follows:

1. Dissasemble the ICM (the thing that looks like a radio), retaining only the mainboard, insulate it, plug it back in and tuck it into the dashboard.
2. Modify the plastic casing of the ICM to hold a single din radio
3. Cut the speaker wires going into the EHU (NOT the power and fibre!!!) Wire up each of the 4 standard speakers to a standard ISO plug that can be plugged into your new radio's harness. Add ground, 12v permanent and ignition live power sources to the plug.
4. Cut the door speakers from the stock amp and connect them to an after-market amplifier. Run RCAs and a remote cable to behind the dash to plug into your new unit. Either leave the stock amp in situ or replace it with a fibre loopback adaptor
5. Connect your centre speaker directly to one of the speaker outputs on your EHU. This will ensure that you retain the car warning sounds.

Enjoy the improved sound quality and up-to-date infotainment system.

Note on the above - the number of amps and speakers varies from car to car, and not necessarily according to trim level. Also the presence of a speaker grille is not evidence of a speaker being present, and the amount of speaker re-wiring needed will differ according to which system is there to begin with.
Also (IIRC) the 4-speaker system will always need one speaker attached to the EHU to provide the indicator ticks.
The 7- and 9-speaker systems have a central dash speaker for this purpose.
The 4-speaker system supplies all speakers from the main unit
The 7-speaker system has one additional amp, the 9-speaker has two.

_________________
I had a meteoric career.
Meteors plummet in flames.
Career is what happens when you lose control.

If you're lucky enough to have the premium system with two stock amps, replace them with a single 4 channel amp.

If you don't have a centre speaker because you have a poverty spec car, put one in! No point doing all the other work just to lose one of the speakers.

Nice one veryone, thanks