Hey mate,
Personally I would design it to handle a bit of current.
I have attached a diagram I drew with part numbers from Jaycar, sorry it's not great but I'm a bit pre occupied studying for my electrical Capstone exams starting tomorrow so not spent much time researching.
I chose Jaycar parts purely based on that fact that they are everywhere and you can order online, you can get equivalent parts from where ever is convenient to you.
Some more information: Before this topic I had actually thought nothing of Glow Plug circuits and the requirements for them, this is all based on research from Dr Google and experiments with current measurement on my own car.
The diagram states to use a 60A breaker, I would try and use a 50A as apparently according to jaycar their 8AWG is only good for 56A (in practice over a short run it will handle more) but it's ALWAYS good design to ensure your Circuit Breaker is rated lower than the cable in order to protect it. but you get the idea. Jaycar only had a 60, 70 and 100A though.
Other things to note: I am led to understand that a thermostat can be used (existing water temp or add one) in line with the push button so that if everything is already hot the circuit will not work.
once again I am not certain of this as I am not a mechanic and really don't know the exact purpose/principals of the Glow plug except to heat the combustion chamber initially. <<<< Someone with knowledge on this please chime in>>>>>
There are some good auto sparkies on this forum who may well be able to offer extended information as to what I have given you and/or correct what I have given you.
Hope it helps somewhat and good luck
Steve