Radio Interference/spark plug wires?

Folks,

I just had the plugs and wires replaced on my 91 Eldo -- also had an MSD 6A ignition installed. Now I get very obvious buzzing on the AM (Delco factory)radio frequencies. You can't hear it on FM broadcast stations, BUT: I listen to my XM radio on an FM frequency through a modulator, and the same buzzing is still audible (but much fainter) in the background. I have checked to make sure the wires are seated (at least the ones I can reach).

Any ideas?

Thanks!