Do you mean that the turtle/rabbit switch is always stuck at turtle and you cannot push to switch to turtle? Or you do not have third speed? One of most headache problem with the M series is the problem with third speed. First thing to do this check. Open the battery compartment door and put the door switch to Normally Close. Pull handle to activate the brake switch. Press in the B contactor, make sure the C contactor comes in. If not please check the small micro switch on top of the B contactor. When the B and C contactor comes in it will switch the power from 12V to 24V (that's why you do not have lift when you go on 3rd speed). Also please check on the mast switch and make sure it is working as again there will be no fast speed whent the second stage mast is raised. Please check on these first and I am sure you will find the problem.
No need to remove contactor panel to access the resistor, just remove the batteries, and you can feel the resistor behind the handle post spring.