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Sounds like your card has failed in a way that has latched the relay to ground. It would be best to get a new card and I would stop using a booster. My shop has seen several two way radios damaged from capacitive discharge type boosters. I would recommend using regular jumper cables from another 24 volt vehicle or disconnect the batteries and charge them individually with a regular battery charger.
Many of the card failures that we see are from reverse polarity or over voltage damage. Some are from persons removing the card with metal tools while the card has power to it. I know the cards are difficult to remove from the holder. But that's the way it needs to be if you want good electrical connections on a card edge connector.
As for the glow plugs, disconnect the green wire and test each one individually with an ohm meter from the GP tab to a good ground. A good GP will have about 1 ohm of resistance. If you see any that have a high resistance or if they are swollen or broken, replace it.
Many of the card failures that we see are from reverse polarity or over voltage damage. Some are from persons removing the card with metal tools while the card has power to it. I know the cards are difficult to remove from the holder. But that's the way it needs to be if you want good electrical connections on a card edge connector.
As for the glow plugs, disconnect the green wire and test each one individually with an ohm meter from the GP tab to a good ground. A good GP will have about 1 ohm of resistance. If you see any that have a high resistance or if they are swollen or broken, replace it.