A lot of the old wiring will be embedded in glops of butyl paste inside the wire "runs". This was normal practice from factory to hold things in place while being assembled.. Luckily, you don't have a lot so if you use the existing wires as a pull, it can go OK.. The key is a steady firm pull which will pull it out of the butyl after a while.. Carefully tape the new wire to one end of the old and pull it through carefully.. It is good to pull a stout nylon string (messenger line) through with the new wire so if in the future you want to run some more, ya have the pull cord already there.. You can hide the ends in the "run", just inside the pull opening.. If that doesn't work, you can use a piece of scrap lifeline from the local boat place to push through and then pull the new wire and "messenger" line through.