If G0d forbid you have a grudge against someone (and you did not rebuke them [enough] at the time--your fault) must you/should you tell them about it before Yom Kippur so they know to ask you for mechila?
It would seem that you should prompt the wrongdoer, or else wholeheartedly forgive them.
The Kizur Shulchan Aruch in סימן קלא - דיני ערב יום כפור says:
וְאַדְרַבָּה, אִם הַחוֹטֵא אֵינוֹ מִתְעוֹרֵר לָבוֹא אֵלָיו לְבַקֵּשׁ מְחִילָה, יֵשׁ לוֹ לְהָאִישׁ הֶעָלוּב לְהַמְצִיא אֶת עַצְמוֹ לְאוֹתוֹ שֶׁחָטָא, כְּדֵי שֶׁיְבַקֵּשׁ מִמֶּנוּ מְחִילָה.
[Not only shouldn't one be difficult to appease but] rather, if the wrongdoer doesn't have any intention of coming to ask you for forgiveness, you should make yourself available to them so that he can ask you for forgiveness.
If he is not made aware of his "sin", how can he ask you for forgiveness?