I'm not familiar with Branson. Glow plugs typically are a simple circuit. Power runs from the battery to a fuse or fuse block, then to a switch, then to the glow plugs. Mine are in series like those annoying christmas lights. Ultimately the other end grounds. Series circuits can be fussy. Nominal 12 volt power should divide equally, 4 glow plugs each get 3 volts. If they are not exactly matched they may not share voltage equally the one with most resistance gets most voltage. All will get equal amperage. Amps times volts equalls watts. Watts are the measure of power. A heater with more power generates more heat. This isn't always a good thing. Too hot it isn't for long, then it's burned out. Replace one, the cycle begins anew, burning out a different one. Eventually even the new one you first replaced becomes the high resistance one and it burns out. If you have one burned out, and yours are in fact in series, not parallel, replace them all.
These are low voltage high amperage circuits. They are prone to failed connections. Use an ohm meter to take each component out of the circuit and test, or use a volt meter or even a 12 volt incandescent bulb with wires to check across each component's two terminals.
