Left headlight 2000 S-10 damaged in accident. The connectors to the low and high beam bulbs survived and they and the wires back to the fuse box look fine. Upon plugging in a new left assembly, the right side low and high no longer works. The relay was changed and the left assembly unplugged and back to the same point where every light on the vehicle front and back are working except the left low and high. The only things not hooked up are the left fender horn and the window washer motor. Those were unplugged to be able to get to the left light wires to check them out. Is it ok to use a test light with the negative battery on one wire and the other tried with all 4 sockets under the headlight relay to see if the volts are there to be sent to the left light?
See if the filaments are broken. Check the bulbs for open circuit.
If I understand correctly what you are saying, the right side headlights only work if you disconnect the left side lights. If that is correct then I would suspect that the grounding for the right side lights has a problem. As a simple test you could tie ground jumper lead to the ground for the right side lights to see if that changes things. If that doesn’t work then I suggest you check for power using your test light to see if it is getting to the right side while the left side is working.
You can always tie the clip lead of the test light probe to the negative battery lead when checking for power to things. Just be aware that the device you are testing may not use that same ground point. So if there is a broken ground connection between the two ground points the test light will work if you tie to the battery ground but it won’t work if tied to a point beyond the broken connection to ground. A bad ground can make it seem that a device isn’t getting power when it really is, and the problem is really with the ground return side of the circuit.