If your factory head unit is suffering from the "ERR3" symptom the cheapest way to fix the problem is to replace the factory head unit with a after market one. You can get the original one fixed but will cost you a few hundred Euro.
There is generally 2 different types of audio systems that you could have in your Altezza:
1. System with external amp located behind the glovebox
2. System with amplifier built into the factory headunit
Most Altezza's to the best of my knowledge have the external amp systems. This makes it trickier than usual to add an after market head unit to your car. There is no widely available after market ISO adapter loom for this setup. There is one company in England that has started producing them but at a price.
to get one of these adapters.
You will also need a facia plate adapter which you can get from Toyota, part number: 55413-53010
. This one is exact match to the finish of the rest of the dash. can can be used with a double or single din. Has a handy small little cubby hole and also has the section that includes the TRC/SNOW buttons incorporated into the facia
If you don't want to spend that much you can get the cheaper Autoleads one HERE
. This is cheaper, but the finish doesn't really match the rest of the dash, it does come with a larger cubby hole for storage and you need to re-use the original plastic part you have that contains the TRC/SNOW buttons.
Then obviously a head unit of your choice. With the above adapter you can fit a single or double din unit.
With this setup you can run the after market head unit with the factory speakers. Or you could replace them all and add some subs and amplifiers like i did for better sound