[JRTJH]

Two suggestions:

First, connect those two wires to a battery, red to + and black to -. The LED strip should light, if not, reverse the polarity. If it works with the wires, you can connect to the porch light at that point and cut off the other end to the correct length.

Second, Nearly all LED flexible strip lights are clearly marked with "scissors" where they can be cut. Look at your LED strip and you should find "scissors marks" similar to the ones in this photo.

If I were you, I'd check the LED's to make sure they work, then cut them to length, then test them again to make sure you didn't mess something up, then mount them and connect them. If you do it in that order, if there's a problem with the LED strip, you can return it to the seller. If there's a problem with where you cut the strip, you won't have to remove it from the trailer (a big mess) and that way, if you do have a functional LED strip through that process, you can be "reasonably assured" that it will work when you get it installed on your trailer.