If you think your problem is with the defrost you can check the heater without a wiring diagram. You will need to have the unit running so that the limit thermistor closes. There is a 3 wire connector, you can tell it apart from the others because it is carrying the Line and Heat and Comp (labeled on the board).First unplug the refrigerator after it runs for a while and is nice and cold. Then unplug the connector from the board and short the line and heater terminals on the connector (not on the board) you will send the line to the heater (what the relay does, when its time to defrost). Then you can use a clamp ammeter and see if it is drawing the normal load current, about 3 A.

You can check the resistance the same way, of course you will have the power off to do that.

If you are going to test it under load, use a fuse of some kind, like a little popper, you can crimp a jumpers using .25" blade terminals. You will need both males and females.

Or you can just use some mechanics wire, or even stranded wire just be careful that it doesn't touch the chassis or anything else.

The wiring diagram is useful however, I downloaded mine form appliance assistant. Of course now he charges $5.00, probably because I downloaded so many Service Manuals.