There is one black wire that is for the second PTO (rear) that is for the lamp on the dash.
Thanks.My guess is that there are more hacks in the harness. Do you have the TM1590 manual which would help you check the wiring out? It would be a slow process but would help you learn everything about the 318. One thing is that the PTO switches have two sides in use, one to operate the PTO, the other side (second pole of the switch) is used in the safety circuit. With the machine being well into it's 30 years a switch can fail (either side independently or both sides). So check that switch operation. It is a DPST switch.
Also depending on your serial number after 1990, there is a brake switch that is used in order to start the tractor.