FM modulators are the best thing since sliced bread.  With the advent of ipods, MP3 players, and other such music playing devices, FM modulators are now a much needed additiona to your music enjoyment needs.  Who wants to spend hundreds of dollars on new car stereos when spending just a few dollars for a modulator will do the trick.  FM modulators are small frequency enchancing devices which transmit the music from your iopd, MP3, or other music playing device to the receive in your car stereo.  It works like it own little radio station right in your car. 

Modulators, like many devices, come in all shapes, sizes, and price ranges.  You should make sure you buy for your needs and budget.  Make sure the modulator you are looking at has the inputs compatible for your device ie: USB ports if needed.  Does your item come with any type of warranty, if not I would not recomend throwing your money away.  You want a seller who stands behind their product.  Generally a lower end modulator prices for those of any quality will be about $20.  Other modulators have additional features such as USB ports, input jacks, and some even go beyond to offer power inverters, a nice feature when camping. 

Also, before I go, keep in mind you get what you pay for in life, while some modulators may seem very inexpensive compared to others, what do you pay for shipping on top of that low price and where is your modulator comming from.  Again, I would highly advise you buy from those offering a warranty.  While the cost may be low, you dont want to buy a cheap product.

I hope you found this small bit of information helpful when making a decision to buy a car FM modulator.