I had previously done a similar mod but with front turn signal LEDs converted to parking light/turn signals. Unfortunately, I never did a write-up with pics, so here's one with my rear turn signal LEDs converted into turn signals/back-up lights. (7440 socket converted to 7443 socket)
Parts needed are 7443 Switchback Amber/White LEDs; 7443 pigtail sockets (standard, not CK); 7440 bulb base with pigtail connectors; your choice of connectors, solder, wire taps, etc. I purchased everything from Autolumination.com.
Sockets and bulb bases:
The 7443 socket needs a small modification to fit into the lens housing. There's 2 small tabs and two large tabs on the socket. It's the two small ones (circled) that get removed:
You can connect your sockets and bulb bases however you like. I used butt connectors. The green (ground) wire from the 7443 socket gets attached to the black base wire. The white (hi circuit) wire gets attached to the red base wire. The red (lo circuit) wire will get tapped into the +12V wire from the oem back-up light connector:
The way the switchback LEDs work, the hi circuit powers the amber LEDs, the lo circuit powers the white LEDs:
Stock wiring harness with everything connected:
Results (passenger side with mod):
I didn't take a pic of the turn signals, but they operate as expected. The particular LEDs I purchased will turn off the white LEDs when the amber LEDs are activated, so if I'm in reverse and my turn signal or hazards are on, the white led back-up light part of the bulb is deactivated.