Headlight Harness Help - Connecting to Existing

[Mthumphrey]

Hey guys... Not much of a electrical specialist here. Maybe you can help. I have a 66 C10 and purchased a headlight harness from LMC. Sounds great, but it came with a 3 prong connector (connects to original harness). The original is a female 2 prong connector (see picture attached). A few questions here.

1. What is the 3rd prong for?
2. Will this at all work if I were to splice it?
3. Or should I just return it? LMC was useless in helping here.

Thanks!
Michael