Horn and trunk upgrade - 2000 Corolla

Yo, so my friend has a 2000 Corolla that she wants upgraded. Her horn is crap, so I'd like to change it for her. Also, she has to use a key to open her trunk and I thought I could put a latch in there, but that sounds tricky. Oh, can I put a light in the trunk, too? where would i run the electrical wires? Thanks. Peace!