If you get inside the alternator you have 3Φ AC at high current, with varying freq./voltage.
I did this once and used a step-up transformer to supply higher voltage for a sound system. Never finished it though. I don't know how the xformer will react to the lower freqs. it will sometimes see.
The wires will be subject to vibration so strain-relieve them wherever you can.
Had a rep here this afternoon, and one of his new products was a car adaptor for running laptops, it provided from 15V to 24V at 6A - and the trade price was only £15 or so.
I Must design and run a Electro motor ( Servo 24v and 4A ) in a car and i found MC34063A ( Set-up , Set-down and inverter for voltage ) but the current limited to 1.5 A ! how can i power it to 4A!?