Albert,

The MARS mod on the 590 is to open up a surface mount resistor.  I think 
it is R900, but you need to confirm that from a reliable source.

While it is considered  a "resistor" it actually has zero ohms 
resistance and acts as a jumper.    Some guys cut it with an x-acto 
knife, or crush it with needle nose pliers.   Others will unsolder one 
end and lift it off the pad so it could be reinstalled later if you 
wanted to restore the radio to  ham band only transmit.
  In any event, you will need sighted help on this one.

73, Steve KW3A