this unit pre dates heat pump models. these are built like tanks far superior build to modern units
unit was built 2001 so your problem here could be brittle plastic in trying to get in and check things
let the unit start and fill, then check how much water is in. try adding another pan full and see what happens. if the water level is too low heater wont come on
you could check wires as they leave the bottom of the door as they can break at this point
after that you need to be good with a meter
Dave