Some of the differences betwen the old ones to the new ones are; the old ones are made of metal and the new ones are all plastic. I dont know about you, but I prefer the old metal ones; specifically for longevity.
This is what I do and it gives life to those old units for another 10+ years.
First: Take your old harness and document its dimensions, lengths, etc. I like to add an extra inch or more to each length for a little more slack when it comes time to plug them in. Remember, when harnesses are made from the factory, they use as little as possible, which puts the wires really tight. How many times have you pulled a light switch from the dash to check out the wiring and the length of slack there only limits you to tilt it downward - and you still have to get on your back to look under the dash to see what's going on.
1. Here is what I have to work with. Old cloth wiring, stiff and ready to fall apart by the touch. A fire waiting to happen. The little rubber rings behind the bulb holders keep the holder from sliding down the wire when changing the bulbs out. These ones were old and crusty so I cut them off and threw away the crumbs. O-rings work well to replace them if you want to add them back.
3. Carefully disassemble the parts from the wire; setting them aside: then strip the old sheathing off to expose the original wire. Be careful not to break off the bulb contact end. (where bulb makes contact at the bottom of the light socket.)
4. Take a small piece of new wire, here I use brown. All aftermarket kits color code the harness for the dash lighting in brown. Typically a 16 - 14 gauge wire. Strip back your new wire and twist it in this fashion as tight as possible to keep the diameter of the twist the same size as the original wire. Dont forget, you have to re-assemble the light socket with the original parts. Dont want any additional issues. (grins)
8. Remember the loom were trying to make. When joining the wires like the original, instead of using those nasty red/blue/yellow butt connectors, strip back the sheathing and twist and solder the wires together. Use the correct size heat shrink to insulate it properly.
9. Our final product of our labor. If you have to add other 'branches' of wire from your main loom, strip back and solder each one. Also, remember to use a larger wire if you plan on branching off mulitiple light sockets.
Plug in new bulbs to your new holders; and connect the end with your proper connector and terminate to source.
Hope this helps; good luck. Post any comments if you follow this to let me know how it went.