You imply that you know the beracha rishona for the fruit salad, and this actually will affect the answer regarding the beracha acharona.
The beracha acharona on the majority of the ingredients in a standard fruit salad is Borei Nefashot. Assuming you eat a k’zayit of those fruits, you should have an obligation to say Borei Nefashot. If you have less than a k’zayit of grapes (and/or the other of the five fruits, including raisins, that require an Al HaEtz), then there is no possible obligation to recite Al HaEtz. The question arises when you have a k’zayit each of Borei Nefashot fruit and Al HaEtz fruit. Which of the berachot “wins out,” or do you make two berachot acharonot?
The main question is how to look at a fruit salad. When one noshes from a vegetable platter, we look at the individual vegetables as separate entities. When one mashes different fruits or vegetables together until the ingredients are not distinct, then we certainly have one entity. In that case, there is a single beracha rishona that is determined by the majority (volume-wise) of the ingredients.[1] The question is: how do we categorize a food, like fruit salad, which is intended to be eaten as a combination (i.e. each spoonful contains a few varieties) but the ingredients are distinct enough to select one at a time if one desires? In this case, a significant machloket exists. The Mishna Berura[2] says that one makes one beracha rishona based on the majority, whereas the Chayei Adam[3] says that one makes separate berachot. The more accepted opinion is the Mishna Berura’s, that one makes only one beracha, but there are some who follow the Chayei Adam’s ruling (or try to arrange things to accommodate both opinions).[4]
The outcome of this machloket has a direct impact on the question of the beracha acharona.[5] According to the Mishna Berura, you look at the fruit salad as one unit, and, assuming the grapes (and other of the five minim, including raisins) do not constitute a majority, you recite only Borei Nefashot. However, if one has the practice to make two berachot on fruit salad[6] then there is an obligation to make Al HaEtz even if the grapes are a minority, as long as he ate a k’zayit of them. Here, the situation is tricky. When one eats separately, but at the same sitting, some fruits of trees that require Borei Nefashot and others that require Al HaEtz, then the Al HaEtz exempts him from saying Borei Nefashot.[7] That is because Al HaEtz is appropriate, on a certain level, for all fruits of a tree, just that this more elaborate beracha was reserved for the five special species of fruit for which Eretz Yisrael was praised.[8] However, that would only exempt one from the beracha acharona on the peaches, apples, etc. But if the salad required a Borei Pri Ha’adama for melon, pineapple, etc. and one had a k’zayit of that component, then he would need to say Borei Nefashot, as well. But again, the Mishna Berura’s approach is the more prevalent one. Therefore, one would just make the beracha acharona that is appropriate for the majority of the fruit.
Let’s end with a little mathematical/halachic riddle. If a fruit salad has 40% grapes, 35% melon, and 25% apples, what berachot (rishona and acharona) does one make, according to the Mishna Berura? The answer is that, regarding each beracha, we must find the common denominator that forms a majority. For the beracha rishona, the apples and grapes join up to require a Borei Pri HaEtz. Regarding the beracha acharona, the apples and melons join up to require a Borei Nefashot. Paradoxically, the smallest component (apples) “wins” twice by teaming up to form a majority.