How old is it? Mine has a linear drive and a control box which has the fluxgate compass inside. The control box, which is separate from the drive, is closed by four cross head screws. There is a red adjuster in the back which will come off with the back, but needs to be replaced in the control pot before the back is replaced.
I think Philip is correct. The red top (which often has the word Nautech on it) is simply glued on and needs to be levered off. The rest is easy.
Getting the contol knob off the bottom (to release the lower circuit board) is another matter, and pulling it off often seems to end in breaking the internal rotary control. Good Luck.
Knob at bottom is removed by picking the little bung out in the middle of it and underneath is a nut that needs some needle nosed pliers to undo,only needs a turn or so, no need to remove completely, knob should then come away easily without busting the switch!