I just installed this relay on my Kubota ZD28 without reading the instructions. On this relay, there are four wires (1-red. 1-black, and 2-purples). On your Kubota starter solenoid, you have 1-red (battery connection) and 1-black wire. When you install this relay, you are placing it in the circuit going to the starter solenoid. Installing the relay is simple:
Disconnect battery (although I installed it live).
1-red wire from the relay goes to the 1-red battery connection on the starter solenoid.
1-black wire from the relay goes to any ground connection on the mower (frame, engine block, etc.).
Pull off (disconnect) the 1-black wire (w/white stripe) going to the starter solenoid.
1-purple wire from the relay connects (slips on) to the 1-black wire (w/white stripe) you just pulled off the solenoid (basically join them). You cannot mix the twp purples wires up because only one connects to the Kubota solenoid black wire (w/white stripe) you just pulled off.
The other 1-purple wire from the relay slips onto the starter solenoid.
Basically, the 2-purple wires from the relay are being installed between the original Kubota 1-black wire (w/white stripe) and the starter solenoid that it was connected to. The other relay 1-black and 1-red wires are just getting power and ground connections so it can connect the complete circuit to the starter solenoid. This is an inline relay to the starter solenoid on the Kubota black wire side. That is all it is. Hope the picture helps. Just follow the wires on the relay.