There is a plastic interior knock-out in back on the port side of my "99. The female plug is just accessible. You can't get a male plug but tin and flatten your copper leads and they will stay in just fine.
One of the leads, red I'm sure, is B+ from the dedicated trailer fuse. With this, you can install a relay type, 4 to 3 wire converter box. All current will be through the fuse.
You can also put in some 8 ohm ballast resistors on the turn signal wires and switch over to after market LEDs.