What should I look for in a Mobile FM Transmitter?

Better than a transmitter...

http://www.crutchfield.com/S-9oDEWuNVIo6/cgi-bin/p...

It's not wireless.

It feeds your audio signal from a headphone jack ( or RCA cables) by FM modulation, directly into the antenna input on your radio bypassing the antenna when on, allowing the radio to work as normal when off. It comes with an On/Off switch that can be used or it can be replaced with a relay powered by the remote wire from the radio.

This is the answer. Totally versatile.

Don't waste your $ on an FM transmitter. I've tried 3 different kinds and all were junk. I finally got an adapter installed with my stereo (about $200) and my iPod sounds FANTASTIC!!! I know it's a little more expensive then the FM transmitters but it's so much better. Also, I can not control my iPod with my car stereo. I just plug it in and leave it in the glovebox. It also charges my iPod, totally convenient. They make these adapters to fit most after-market decks as well as newer factory systems. Check with a good automotive audio place, they'll be able to help you tons.

Go with Sparkys answer, I have that set up and it works great, its pretty simple to install depending on your knowledge of wiring up stereo equipment. Basically it is a FM transmitter, but it cuts out your cars antenna leaving more stations open and free from interference.

One option is to get an FM transmitter that has an option to be run by battery, and car power. That way you are not limited to just using it in the car.

http://www.radioshack.com/product/index.jsp?produc...

Or you will want to get one that charges the I-Pod while you play it.

http://www.radioshack.com/product/index.jsp?produc...

Preferably has its own power so it doesn't need juice from the iPod itself. I think MonsterCable has a set that's integrated with the car power plug. You also want digital tuning if available and more than a few choices, preferably 10 or more.