for reference just installed a new transmission in my 95 ex civic. tranny is from 99 ex.

okay when the headlights are off everything is great. when you turn the headlights on everything is great. but left headlight randomly turns off by itself and when that left headlight turns off the speedometer will stop working. then as soon as the headlight comes back on, speedometer comes back on. well???????

example: wifes driving with the headlights on, so lights are on and left and right light are working and speedo are working, then she hits a bump and the left light and speedo turn off. so shes driving and if she turns off the lights the speedo works if she turns them back on and only the right light is working then the speedo isnt working. then she hits another bump, bam left light turns back on and speedo turns back on.

speed sensor?!?! wiring?!?! possessed?!?!

 

i say check check your ground wires first
in your engine compartment

 

that was the first thing i checked, anyone else?

 

well if this started happening right after you installed the new transmission i would say trace the wires from the left headlight back and see if there might be any spots where the wires could be shorting each other out might have pinched a wire when installing the tranny or might be some loose conections thats where i would start