Can't find the original post, but looking through my eBay history, these were the parts I ordered:

• https://www.ebay.com/itm/374251716700 (Female 7443 socket that fits into the tail light housing. Even though the Volt uses 7440 bulbs, only the brown 7443 sockets will fit. 7443 bulbs are the same size as 7440 but have extra pins, so you'll need to cut off the red/blue wire and cap it with some heat shrink)
• https://www.ebay.com/itm/394315693134 (male 7440 plug that plugs into the existing socket)
• https://www.ebay.com/itm/252828141931 (150ohm 3W resistor)

You'll have to make up an adapter for only one side. You're going to connect the black wire from the plug and the green/yellow wire from the socket to one end of the resistor and connect the red wire from the plug and the black/white wire from the socket to the other end of the resistor.

The end results are going to look something like this, but with a tan connector instead of a grey one, a black/white wire instead of the red/blue one, and a much smaller (and cooler) resistor.

If you have a soldering iron I would just solder the connections and cover the whole resistor and the solder joints with some 3:1 marine adhesive-lined heat shrink (I use this set from Harbor Freight). If you don't know how to solder you can use something like this and a heat gun to make the connectors on each end of the resistor and then cover the whole thing in heat shrink or electrical tape.