I've actually seen worse bottom panels (but very few.) Remove the bottom panel, pull up a comfortable chair and take your time. When trying to figure out after-burn wire color coding, Steve Young of Pinball Resource taught me neat trick: pull the cabled bundles apart and trace to the point where the clean insulation of the wire in question can be exposed. Most of the blackening occurs to the outside surface of the bundles, so they are quite often protected from heat and dirt exposure inside (somewhere down the line) where you'll be able to see the original colors and patterns. Buy lots of heat shrink tubing in long lengths. You'll need it.