Termini is a bear to find (Radisson Blu) and you have to go all the way to the top floor. Turns are very tight and narrow (didn't help that while I booked a FIAT Panda or equivalent, I ended up with a 500 XL). If you choose it, you can only approach the Park Giolitti garage heading NE on Via Mamiani and turn right on Via Giovanni Giolitti when the street ends in front (or rather, the side, of Termini. Entrance (small rental car markings) is immediately to the right.