You can find the wiring diagram here..
Parts and Diagrams for Cub Cadet 1525
but you really do not have much wiring to wring out... typically voltage drops are in bad connections either at or in switches... check for heat damage or corrosion, and check all grounds... The PTO switch itself is a good spot to check for internal resistance, that will cause heat and that will increase the voltage drop.
The clutch with no load on it (deck drive belt removed from clutch) the whole assembly will usually spin, that is not a problem... :smile:KennyV