Built a preamp, getting radio fequencies.

I'm a great newbie to soldering and doing electronics.

I built a simple preamp, after having just soldered the thing, it worked fine. I then assembled it inside a plastic toy-car.
Now it keeps picking up Radio frequencies.
I am testing this preamp with my bass guitar combo.

How do I get rid of the preamp working as an antenna in an easy way?

Try lining the car with foil or do what ever you can to shield it. Add a ferrite bead to the power cord to block RF coming in that way.