As Karna was very powerful and was also the son of Sun, did Krishna ever try to help or teach dharma to Karna? Did he try to make him a good person or protect him from evil thoughts?
Yes, Krishna did.
In-order to perform his duty of intercession completely, Krishna teaches and tried to help Karna. But Karna, accepting Krishna's words, explained his reasons for not implementing the advice of Krishna.
Although Karna accepted all the words of Krishna, he cannot able to union with Pandavas due to several reasons. And he is aware of the upcoming destruction yagna.
Sanjaya said, 'Hearing these beneficial and auspicious words of Kesava, Karna worshipped Krishna, the slayer of Madhu, and said these words,'Knowing (everything), why dost thou yet, O thou of mighty arms, seek to beguile me? The destruction of the whole earth that is at hand for its cause, Sakuni, and myself, and Dussasana, and king Duryodhana, the son of Dhritarashtra. Without doubt, O Krishna, a great and fierce battle is at hand between the Pandavas and the Kurus which will cover the earth with bloody mire. All the kings and princes following the lead of Duryodhana, consumed by the fire of weapons will proceed to the abode of Yama. Diverse frightful visions are seen, O slayer of Madhu, and many terrible portents, and fierce disturbances also.
One of the interesting reason is that Karna wants Yudhistrira only as a king. It can be clarified form the following words of Karna
The Pandavas also, obedient as they are to thee, will, without doubt, do all that thou hast said. Thou must, however, conceal this our discourse for the present, O slayer of Madhu. Therein lies our benefit, I think, O delighter of all the Yadavas. If king Yudhishthira, of virtuous soul and well-controlled senses, cometh to know me as the firstborn son of Kunti, he will never accept the kingdom. If, again, O slayer of Madhu, this mighty and swelling empire becometh mine. I shall, O repressor of foes, certainly make it over to Duryodhana only. Let Yudhishthira of virtuous soul become king for ever.