Ipod Modulator or something better for 2001 Prius

Do you have a cassette player or CD Changer? The cassette adapter would be the easiest choice with the best sound quality.
The Griffin Tech Smart Deck seems to be the best, but people with Gen 2 Prius have said it doesn't work. I have no idea if it will work in a Gen 1 Prius.
It appears the iPod2Car should work with the radio in a 2001 Prius. If you have a CD Changer, the iPod2Car could be a problem since it emulates a CD Changer.
You can use the Vais Tech SLI but you would need to buy the correct cable for your radio.
This is an example:
http://store.yahoo.com/logjam/pietoyfmm.html
Vais Tech is small and it isn't cost effective for them to make this cable.

 

An option that isn't talked about much on here that I quite like is the wired FM modulator. This is something that is wired directly into the antenna of the radio so that you don't have all of the problems that wireless FM modulators typical create (i.e. bad reception if you're near a strong FM source on the same frequency). I've been in a couple of cars that have them, and been pretty impressed with the sound quality (i.e. nothing like the straight-into-the-amp solutions, but still pretty good).

See this link for a pretty good-looking FM modulator.