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What is the battery voltage? Has to be above 12.5 volts. Also pto will not engage if voltage is too low.
How old is battery? Have you cleaned battery terminals and tightened the cables?
The pto switch, seat switch and brake switch all work together to complete the start circuit. Have you checked the seat switch and harness, it is the one that causes the most no start issues.
If you trun on key, and flip pto switch, does the pto clutch click closed?
So pto switch is good. Now we need to trace why the start circuit is not complete. Did you check the continuity between the switches?
Here is basic Snapper wiring schematic. Yours will be very similar. Switches may be good, but are the wires between them good?
Broken wire or a bad connection at one of the switches, would be my thoughts.