how to fix up bulkhead connector?

Silver loaded dielectric grease will usually restore very mildly corroded connections to full integrity, and make them even better than new. But it won't fix burned up ones for sure. You can find that on eBay and at many industrial supply houses.

And, there is a product called Cool-Amp that is rubbed onto metals to give them a silver plating. But the Cool-Amp in particular is pricey, so is not a good choice if you are doing just one car. But, you can restore a lot of older auto electrical parts to better-than-new condition with it; I restored the headlight switch in my '62 Dart with it and it is as good as new now. This car is so original that I am trying to restore/maintain as many original parts as I can.