I purchased the set of '99 taillights for my '03.
Haven't put them in yet because I have not located the sockets but this is what I wanted to do to add separate amber directional lights.
1) Cut the existing wire from the brake/directional bulb.(Leave enough wire for splicing another wire to it)
2) Attach the cut wire from the brake/directional to the new directional socket.
3) At the brake light switch, disconnect the wire going to the brake light.
4) Run another wire (16 gauge) from the brake light switch to the old brake light location at the rear of the vehicle.
By moving the existing brake/directional wire to the directional location it lets the bulb function as a directional through the existing system.
Disconnecting the existing wire at the brake light switch stops any brake signal from going through the existing system; you have just eliminated the existing brake lights.
By running a new wire from the brake light switch to the brake lights you add a new signal path for the brake lights only.
NOTE:
Probe all of the wires ahead of time and mark them and then double check that you have the correct wire before cutting.
You might want to disconnect the existing brake light wire at the brake light switch and verify that the directional lights still work and the brake lights don't, that is before cutting anything.
Preparation is the key, take your time and do it right.
Solder all of the connection but only after you have a good mechanical bond and you have tested the circuits.
Use adhesive type heat shrink to keep things water tight on all soldered connections.
If anyone finds fault with this please post the correction, I want to do it also and times a wast'n.