Use a pulse width modulator (PWM) and a big MOSFET driver. I'm including a schematic. With the potentiometer, you can adjust the duty cycle (power dissipation of the glow plugs) from zero to 100%. There are also dedicated ICs that can be used for PWM. If you use the MOSFET I've shown, its maximum average power dissipation will be about 3.7 watts, so you'll need a heat sink.
The LM393 is shown as U3 and U5, but it is a dual comparator, so you only need one.
You might be able to use a noncontacting IR thermometer to measure the temperature of the glow plug. Otherwise, I'm not sure how you would do it. It seems like any sort of contacting sensor, such as a thermocouple, would be a heat sink. I have a cheap IR thermometer that I bought at Radio Shack. Its upper limit is 400F, and I doubt it focuses well enough to measure a glow plug accurately. If you already know how to measure the temperature, can you share the method with us?
I edited the schematic. The old one had the glow plugs connected to the positive battery terminal, driven by an N-channel MOSFET. The glow plugs are now connected to the negative battery terminal, driven by a P-channel MOSFET.