You are thinking about two totally separate issues. The doghead relay mod (named after one of our members) is just replacing the starter relay under the dash with a heavy duty one, there is no kit. It takes about 20 minutes to do the mod. Look at the stickies at the top of the CUCV section of the forum and one of them will be the instructions for the mod.
Burning out the glow plugs is a common problem related to the glow plug resistor mounted on the firewll behind the air filter housing. It is designed to drop the 24 volts down to 12 volts so the glow plugs get the proper voltage to them. The problem exists when you have a glow plug go bad, the resistance of the ststem changes and the remaining plugs now get too much voltage, and progressivly burn out. As each plug burns out, the rest get higher and higher voltage with the fewer good plugs left in the system. Most people simply bypass the resistor and feed 12 volts directly to the glow plug relay.