I changed the head unit in my tezza also.
After bein quoted €400 to get the new radio fitted i tackled the job meself.
It cost me €35 and didnt cut any wires.
A wiring lumb for a toyota MR2 from auto leads fits perfectly. Just plug in and away it goes.
My car is 99 and doesnt have a seperate amp.
But i think fresher tezzas form maybe 01 do have but think it should still work if u bypass it.