In my 40 year old
Trillium and in my old Bolers (Boler 13 now
sold and
Boler 17), the area inside the camper behind the side marker fixtures is uncovered bare
fiberglass surface. When I replaced the side marker fixtures on my
Trillium, I drilled a hole straight through the fixture AND camper shell, then pulled the new fixture wires inside the camper and then crimped them to the original camper wires. Easily done.
Living in the Northeast, I decided to locate the wire splices INSIDE the camper to protect them from road salt and precipitation moisture.
Edit: Again, living in the Northeast, my old side marker fixtures were badly corroded (wire terminals, bulbs, sockets). I was constantly cleaning and spraying the old fixtures to keep them working. Hopefully the new LED fixtures will perform better.