Not necessarily a wire. I've had bulbs that had broken internally do that.
On an "older" truck, it could be anything. If the electrical shop didn't find it, I'm thinking one of the following:
1. Bad socket
2. Bad bulb
Any bulbs "out"?
Most stuff on the truck, you can "thump" (like the switches) -- but the bulbs are harder unless you pull the sockets out. If you're willing to do the work, you can take your bulbs out (in the socket) and give them a "thunk" and shake them and see if they cause the fuse to go.