Help! electrical gremlins

[69heavychevy]

for some reason the fuse that power my dash light keep blowing. i know the culprit but dont why it doing it. i have a 800 watt amp that i use to power all the speaker in my truck, two 4" tweeters, two 6x9, and a 10" sub. everything was find and dandy until i installed a sunpro tach. two week after the install i blew the first fuse. didnt think much of it. they kept blowing but i noticed it only does it when i have the headlights on and my stereo up loud. yes i understand that puts a good size load on the electrical system, but take in mind it had never done it before until i hooked up my tach. with the tach hooked up it would take seconds for the fuse to blow, unhooked will take minutes, and with headlights off wont blow. i tried putting a 10amp fuse, BIG!!!! mistake. i had never heard a sizzling sound followed by tons of smoke come from under the dash!!!