whirlpool wrn38r iced up and leaking onto the floor.

Second time it’s done this. Defrosted it over night and it’s been good for a month but now it’s iced up again. We’ve checked the drain pipe down to the tray at the bottom and it’s clear. Is there anyway to get into where the ice is? I’ve googled a bit and read somewhere that there’s a gromit that has to be changed???? Or is it a thermostat problem? Fridge is 15 years old.
Any advice would be much appreciated.