Hum in the Cassette/Radio

Hi everyone!  Just installed an "in-dash" style Cassette/Radio in our TM and it hums when the TM 110 volt plug is connected to main power.  When I disconnect the plug and run off of the TM 12volt battery the hum goes away.  I've tried the white ground wire in the stereo wire bundle inside the mounting hole (above the fridge), the ground of the 110volt plug beside the fridge and have also tried connecting the stereo ground wire to a spot under the TM on the steel frame to no avail.

On a  separate (but related) note...how have you removed the speaker grilles?  I attempted to remove them but stopped when I felt I needed to get my allignment tools (hammer and a flat bladed screwdriver) out of the toolkit.
Any suggestions would be appreciated.
Thanks!
Mark