noise problem

I recently installed an fm modulator into my car and it works great.  However, I'm getting a noise problem when I try to use it in conjunction w/ a power inverter.  I get the noise problem when the inverter is on and something is plugged in.   Does anyone know of a solution to this or do I have to just forget about using the inverter when using the fm mod?  Thanks.

I see.  Well, I'm using an inverter that just plugs into the cigarette lighter so it is probably not being grounded that well.  Any other thoughts on this?  How would I improve it's ground connection?

Also, I was testing the inverter with my laptop.  I plugged my laptop into the inverter and the audio output into the fm modulator.   That was when I was getting the noise.  

Thanks for that reply.

The FM modulator is grounded to a spot where toyota grounded another piece of equipment.  It looks like it is connected directly to the floor of the 4runner.  It is basically the metal case where I screw my deck into.

And, yes, I did have my car running when I was doing this.  Everything works fine even when the inverter was plugged in.  Only if I plugged the laptop in would the thing go crazy.  I wouldnt even have to plug the audio output to the laptop.  All I have to do is touch the end of the cable with my finger and I get the loud humming.