If you are adding a relay to your tractor for any kind of electrical project, you will need a TVS diode for the circuit. This diode protects the contacts from arcing on the switch that controls the relay coil. Arcing will damage the contacts and reduce their useful life.
In the GTT Technical Library, eepete provides a detailed technical explanation (in layman's terms) of how these diodes work and are used. LINK HERE.
This is what the diode looks like:
I recently installed a horn relay. The twin horns required 14 amps and I wasn't sure a push-button switch this large would fit on my dash, so I used a smaller, low-current switch to control a relay with 20 amp contacts.
For my circuit, I selected an SA18CA TVS diode. This diode clamps any transients starting at 18 Volts. Normal voltages in a tractor don't exceed 15 volts, so the diode won't turn on for anything except for unwelcome transients. This diode is bi-directional, which means you can connect either end of it to either end of the relay coil. For a data sheet on the diode, simply type "SA18CA data sheet" into any internet search engine.
You can see this diode in a real circuit diagram in this post. In the photo below, you can see one of these diodes installed on my horn relay.
When I strip the wires that will be connected to the coil, I take the stripped insulation and put it on the diode leads, leaving enough exposed to crimp. Then I crimp the diode's leads into the same quick connects (along with the wire) that are used for the wires going to the coil terminal.
This little component, in small quantities, can be purchased from someone like DigiKey for $0.54 each. But the shipping will dramatically increase the cost unless you need some other stuff that DigiKey offers. I found a deal on E-bay where I could buy 10 for $5.68, including shipping, from a surplus company. (I could buy 10 for less than I could buy 1.) I needed only 1 and I'm going to keep a couple for possible future projects, but the other 7 diodes need a good home. If you have a need for one, please PM me with your address. I'll put it in the mail for you.