I fully rewired mine the other day, exact same toilet. Finding out how to get the switch unit off from inside the door (on outside) was not fun, until I sussed it! 4 screws dropped the unit out. I used a meter to check for continuity when switches were pressed. There are two reed switches, one is a safety so the flush won't work when the loo cassette is removed. The other is the push switch to allow power to the pump. I have to say, if I was to start again, I would seriously look into whether I could convert my toilet to the manual pump. Those things work lovely and no silly electrics to go wrong. How hard is a brief turn of a handle versus a brief push of a troublesome switch?!
Anyway..... I found the 'safety' switch on mine was faulty so I just removed it, soldered the wires together so now if the cartridge is out, the flush will work. Will serve someone right if they forget
. I replaced all the wires as they had bad oxidation/corrosion on them. It took me a very long time to figure out that even though I was seeing 12volts, the pump (new) wouldn't run, and then I realised the corrosion was limiting CURRENT, even though the voltage appeared to be there. That's my conclusion anyway, and replacing the wire with decent wire (tinned copper conductors, although you needn't do that, I just had some spare) solved it. Now working very nicely.
Reaching in to get at those 4 screws was probably the hardest part of the lot. If you can get that unit off, worst case you can just buy a replacement. Or you could replace just the reed switch if that's at fault. Not sure if this helps, but I feel for you either way