heat. if it gets too hot from the excess load it can burn up connections. 5 amps is alot on a dc system.

a 15 amp fuse will work fine as long as you use a 14 gauge wire or bigger. I would use 12gauge personally with a 15 amp fuse with in 10in of the ignition switch

What's the proper way to connect it to the wire in the ignition harness? Just want to make sure that connection will support the amperage.

inline. if you dont have inline strippers, take a razor blade and cut the insulation away on the constant wire on the ignition harness. Expose about 1in of wire. Take your new lead and wrap it around the exposed wire tightly and then solder it. If you dont have a soldering iron you can just tape the connection. Remember to wrap the tape in the same direction you wrapped the wire and if you didnt solder it, after you tape put a ziptie on the connection as tight as you can so there is no chance of the connection coming loose.

Cool, that's pretty much what I've done in the past, just wanted to be sure

Well then there ya go man. Do it up!!