Is it better to build a box for subs or let the trunk be the box? I heard the trunk is better because it is big. As long as it doesn't rattle and buzz that is.
It depends on the speaker, but a trunk is bigger than needed for most 10's and 12's. On the other hand, sealed speaker boxes can be larger than needed. Ported speakers can't, so if you use your trunk as the box, don't port it unless you know for sure the speaker can work in that large of a box.
If you use the trunk as the box then you can't tune the enclosure for the best possible response. You also can't just toss any old driver into a trunk mount or again it won't work (I assume that by trunk mounting you mean mounting the driver through the rear package shelf; all driver require some sort of baffle top separate the front and rear driver waves). The best bet is a tuned enclosure mounted in the trunk, or a more exotic kick panel arrangement, just as long as the enclosure and driver are properly matched.