Your problem is to first find the defrost timer. They hide them all over the place. Sometimes inside the box and sometimes outside. You can advance the timer manually, clockwise. Advance slowly. When you hear it click, the compressor should turn off and defrost heater turn on. Don't advance it any more after compressor turns off. Measure current with ammprobe. No current means, bad switch in timer, switch is a SPDT, something wrong with heater or defrost switch.
Good luck finding the timer. Timer could be a fancy electronic switch. Those could be expensive. Might be time to wait for a black Friday sale. Many appliances will be on sale soon. Recently I purchased a 22 ft Frigidare side by side with ice and water in door for around $800 from Lowes.