I tried to use a small power invertor to power my AM/FM radio in my work truck as no radio is provided and wanted to avoid batteries. FM works great but AM is only noise. Even tried my Bose Wave Radio with the same result. Would an invertor that says "modified sinewave output" give me a different result? I have had several AC/DC radios over the years but there aren't many available anymore and the quality is pour i.e.. Radioshack. Thanks for any help.

Not sure about not available, I just bought three that have either 110 or 12 volt adopters available, ( ebay) all new, they did not come with the adopter but have the hole for it, and you can buy one that fits dozens of sizes at walmart.

But yes the sinewave most likely could have an effect, I am assuming this for the same reason there can be some computer issues runnign off a generator also.

GO VIDEO PORTABLE DVD PLAYER-AM/FM RADIO, CD'S, & MORE! - eBay (item 190188703791 end time Jan-12-08 13:05:10 PST)

Is just one example,

I would plug Your AC/DC radio into the Cigarette Lighter or add 12 outlet or connector, Your battery in the truck will last longer. Conversion and inversion have losses.
Plus DC is Silent.

Thanks for the input. Does anyone have an idea about the modified sine wave converter and interference?

AC is Suceptible to Motor Noise and Powerline interference.
If you insist on an Invertor, try:
Pure Sine Wave Inverters
The way you propose is truck battery to inverter(12 volts DC to 120 Volts AC)then radio power cord to 120 volt transformer, and in the radio it is converted back down to DC voltages. I take it, you have a good battery.
Sounds like you want to make noise, then eleminate it.
I would start with no noise, and enjoy the Music.
Sorry if it sounds like I'm bent on DC to power this.
Also this should help:
Modified Sine-Wave Inverter Enhanced