Repair of ground wire within the harness for Parking / Signal Light

I have restored the signal lights to my 1971 Thunderbird, and even found new gaskets for the housing ( @ Eckler’s ). One of the lights works perfectly, the other has an ‘issue’. The two wires which carry current for the 1157 bulb ( one for the parking light and the other for the signal light ) work fine, as I by-passed the ground wire in the harness, and they both illuminate within the bulb while it’s seated in the lamp body. I had checked for continuity with my multi-meter before I did this, and there was no problem with the white wire / blue stripe and the brown, but the black ground had no continuity. I then cleaned the black wire ( female ) connection inside the blue plug with a small wire brush ( showing clean copper ), and yet still no current.
It must have a problem within the blue plug or my guess it has become severed from the metal bulb socket, within the black rubber boot.
So has anyone repaired the ground wire within the lamp body harness ? I know this is a T-bird, but the construction is identical to my Cougar.
I’m thinking opening up the boot to reconnect / solder ( ? ) the ground wire and then using shrink wrap ( with the water-proof seal ) to repair the boot ?
Any ideas / help would be gratefully appreciated !

On repairs like that, I’ve had good luck with star brite liquid electrical tape.