Gentleman, thanks to you the fridge is going again. (Turns out it was a Westinghouse, not a Whirlpool, sorry about that). I disconected the wires to the defrost timer and the defrost heaters and it has run fine for three day with no problems. MikeD74T, as you said, the timer and heater was behind the freezer liner. I got into it and disconnected and taped up the wires. Theres no fan under the fridge, but it does have a heater under the drip tray and I'm pretty sure it was this one (not the one in the freezer) than was shorting out, but getting into the freezer liner enabled me to disconnect everything to do with the defrost cycle.
I'm a happy man. Fixed for free, and no need to find a replacement for a fridge that only gets used a few days a month. Thanks again.