The first thing I'd eliminate is if the glow plugs are being over-volted. At the relay/controller unit there is a wavy Z-shaped shunt resistor that connects the outlet post of the relay to a post on the controller. The end of that resistor at the controller post is where the wiring harness for the glow plugs themselves takes power. You'll need a new set of functioning glow plugs but once they are installed then connect the + of a DC voltmeter to that post (the bottom connection of the Z-shaped shunt resistor) and the - to a good ground on the motor and then hit the glow plug switch.
You should see the voltage start at around 8 or 9 and go up in a few seconds to around 10. It should creep up to maybe 10.5 by the end of your 10 second cycle. If the voltage goes higher than this, to say 11 or higher, then you are likely over-volting the plugs.
Also, if the shunt resistor is missing and someone simply connected the glow plug harness straight to the output terminal of the relay it could also cause this.
Are all your glow plugs reading infinite resistance or just a few of them?