IF there might be a thin layer of spray undercoat/sealer along the outside seams, where the two halves meet, cut through it with a sharp knife, carefully, before trying to separate the two halves, for good measure. Plus what @Big_John mentioned.
On a separate thread I outlined how I found and fixed it. It was a combination of burned and too small a conductor feeding the instrument, parking & stop lights hidden under the bundle on the back side of the fuse block. The headlight intermittent was the bulkhead corrosion.
That connector needs a rewire. If the housing isn't damaged, clean it up after removing the terminals and reuse it, otherwise I have seen them on ebay. The terminals can be removed with long nose pliers, gently squeeze the wide end while pulling on the wire from the back side. Write down what color goes in each location for reference. terminals are available on ebay and at delcity.net. I use this with heat shrink tubing: from ebay
300 PCS ASSORTED UNINSULATED BUTT CONNECTORS WIRE SPLICE ALL SIZES 22-10 GA USA
A micro butane torch makes quick reliable connections when you solder them. Or you can do a complete rewire of the affected circuits. A soldering iron will help with soldering the terminals. Makes them 100% reliable, no plumbing solder, electronic only. preferably 63/37 lead/tin.
The firewall side is a bigger challenge. You will need to get the entire connector out of the firewall. The metal clips at the top and bottom will allow this. Once out the bad terminals can be removed with a small flat blade jewelers screwdriver. There is a small hole above the terminal on the engine side. Stick the screwdriver in and give it a gentle twist while tugging on the wire from the back side. R&R one terminal at a time to not get them mixed up. You're working under the dash so this part isn't easy.
Pay attention to wire size when doing this. Larger is ok, smaller is not.