There are a couple of other options, too.

1.  Get an extra taillight.  Remove 2 of the contacts from it and put
them where there presently are none in your working taillight.

2.  You could purchase 2 single filament bulb holders, remove their
contacts and springs, drill a small hole in the bottoms of the "empty"
bulb sockets,  feed the wires from the contacts through them and run the
wires externally.

Mike