Electrical help needed, underdash wiring is a disaster

The brake lights on my dads 65 fish stop working so I checked the wiring at the tail lights. Eventually ended up under the dash and it's a mess of poorly added new wires with heat shrink and butt splices etc. The fuse panel seems to be bypassed with inline fuses added.
Other than buying a new harness for $600 plus dollars which I can't afford, what's the best way to clean this up and make it safe and reliable? Trying to figure out where to start..