The turn signal socket on my MK1 VW Rabbit Pickup (part# 179971921) has completely rotted out. I thought I would be on the hook for ordering a rather pricey replacement through either e-bay or one of the VW specific aftermarket vendors but thanks to some research and hints I’ve discovered that it was a shared design with Ford of the mid-1970’s to early 1980’s.
The Ford specific part number is D1FZ13234A but what you’ll end up wanting to search for the Dorman aftermarket replacement (part# 85898) which can be easily had at Walmart, RockAuto, and Amazon for as little as third of the price from a VW specific vendor. This should also work for replacing any of the other sockets in the rear tail wiring of the Caddy. I’ve ordered some cores from AliExpress to try and fab something up with the rotted out socket. In short, replace VW signal socket 179971921 with Dorman socket 85898.