DID I DO SOMETHING WRONG CONNECTING THIS WIRE?

HELLO, ive recently replaced my friends fuse holder,fuse and got a new power 4gauge wire to run to the battery, since his original fuse holder blew up on him. what I did was fairly simple, just remove the old fuse holder, replace it with the new one, and connected the 12v wire to each end, as its suppose to be, now 4days later, hes complaining that his lights in his car turn on, and saying that i messed up his car, ive done this plenty of times and never gotten any problems. so is it me that did something wrong? or is it that something else is wrong?...he has 2 12' kicker comp. with a 250 watt rockford mono dblock amp.