What do you all use for trailer light hookups?

[thedudeabides86]

Hi.

My father in law has a sweet sea green 73 Starcraft fishing boat that he got from his father in law. We're working on getting it ready to go to the lake after ten years and it would look sweet behind the ol truck. What kind of wiring harness was factory back in 1972 on the C20's? The trailer has a flat 4 way harness and it looks like the underside of my truck has a round sort of plug on it. I haven't dug deep into it yet, as I thought I'd check with the fine folks of 67-72 chevy trucks. I appreciate the feedback and advice. Thanks!