Fuse Help

ok, so yesterday i noticed i had 2 blown fuses. a 15 and 20 amp. i replaced the 15 one like nothing, but then when i went to replace the 20 amp one, it had a constant big RED spark. i immediatly let go. i never fully plugged it in. and i look at the fuse and it blew, and the tips were literally melted.
i was in shock. i tried another one and same thing happened. any ideas???? im not too experienced with fuseboxes or interior wiring.


EDIT: the fuse is for oil light and passing beams. just some info in case of anything. Thanks guys.
 
AutoZone wiring diagrams

http://www.autozone.com/servlet/UiB..._us/0900823d/80/16/71/3c/0900823d8016713c.jsp for 79-88 model Mustangs

http://www.autozone.com/servlet/UiB..._us/0900823d/80/19/59/5a/0900823d8019595a.jsp for 89-93 model Mustangs

http://www.autozone.com/servlet/UiB..._us/0900823d/80/1d/db/3c/0900823d801ddb3c.jsp for 94-98 model Mustangs

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You should check your bulbs themselves also. (Unplug 'em and try putting the fuse in.) Make sure they are'nt the problem. Then check the high beam switch before you go cutting your harnesses apart to find shorted wires. Also check and see if the power wires to your lights show any resistance to ground. (Using a multimeter, with the fuse out.) If you find one, thats most likely your culprate. Check for anything unusual by your oil sensor too.