20 amp fuse keeps blowing on my 4.3 Merc

[stoth72]

i have a 1986 thundercraft with a 4.3 Merc in it.
the main 20 amp fuse blows as soon as i turn the key to the on position.
i'm thinking it is probably a short some where and was wondering where to start.
since it blows in the on position does that narrow down where i need to look.

 

[alldodge]

My first guess would be the condenser or coil. Remove the + side off the coil, also could be your key switch or gauges. Every gauge will have a purple wire going to it, pull them loose and one by one reattach.