you don't list which model Deville you have -
and you don't specify which radio you have -
but assuming base Deville and base radio -
copied below from the 2003 Deville Owners Manual - page 3-83 -
SETTING THE TONE -
TONE: Press and release this button until the desired
tone control (bass, treble or midrange) appears on
The radio keeps separate tone settings for
each band, preset and source.
- LEVEL +: After selecting the desired tone control,
press the plus or minus symbol on this button to select
the desired level.
To save the tone settings for your presets, press and
hold the numbered pushbutton for the desired preset for
more than two seconds until you hear a beep.
also check page 3-103 RADIO PERSONALIZATION WITH HOME AND AWAY FEATURE -