First thing i would do is take ALL those positives out of the battery and connect them to a Blue Seas 12 gang fusable box. If they are any fuses on the wires(+) going to the battery, throw them away and cut the wire before the fuse and connect to the Blue seas terminals. Get self some red gauge wire and black 8 gauge wire and connect the negative from your battery to the Blue seas negative post and the red wire from your battery switch(if you have) to the positive post of the Blue seas. Now you can check if your horn and other stuff work. If they don't, shut down the battery and check for continuity on your switches. If you have continuity on ON and no continuity on OFF, your swithes work. Problem is your accesories. When you install the radio, make sure the memory wire(usually orange) is connected to the battery all the time so you don't loose time and station pre sets.
This is the Blue seas fuse block
http://bluesea.com/category/5/21/productline/126
This is the battery switch
http://bluesea.com/category/1/productline/2