15 amp fuse keeps blowing out any thoughts?

[monkeyslide17]

89 s13 coupe well goto the gas station and found out that my brake lights are not working, so pulled out the light tester and come to find out the fuse is blowen. all the other fuse's are fine, just the brake lights. well i replaced the fuse and tapped the brake and the fues pops right in front of me, change the fuse from 15, to 20, to 30 and all pop right in front of me again. roll from the gas pump to an empty spot and what do you know put in another 30 fuse and its works, roll more then half way home and then it blows out on me again. any thoughts?? im confused at this point . I have not done anything to my car that might have crossed any wires, have not changed anything.