Headlights not working. replaced headlight dash switch assm. headlights work now but dimmer slide switch turns on dome light as well as dimming dash. reinstalled old unit same problem with dimmer thoughts?

The tail lights work off the headlight switch, and there is a fuse for the tail lights. Make sure the fuse is good, first, a 15 amp (blue) fuse in the instrument panel fuse block. Then, you might have a bad headlight switch.
So you DO have dash lights, they just don't dim from the dimmer switch? In the wiring diagram, there is a green wire from a "tail light relay" (in the "Junction Block-Right Instrument Panel") that carries power to all the exterior lamps, as well as having a splice to the Instrument Panel Cluster (illum. bulbs), and to the lamp dimmer, and to the radio bulb.
To do any further testing, you would need a voltmeter or a test light, so you could chase down this power loss. Good luck.

If you have a light control module then that is probably failing. However if the dome light is activated by small switches in the door pillars it is possible that the contact points may be rusty or have a bad earth.

Please clarify:
Half of your dash lights are not working?
All your dash lights are dim?
IF all are dim, to the right of the headlight switch, there is a dimmer 'wheel' rolling up = max illumination, until it clicks to turn on enterer dome light.

check battery, also alternator

Replace the dimmer control. The door lock switch needs to be inspected. Repair may be possible.

check your dash light dimmer switch. it is usually by the headlight switch, it might of gotten bumped accidently. if this doesn't fix your issue then i need to know if all dash lights are out or just the ones in the instrument cluster.

The only possible location I can think these circuits cross is at the headlight switch.The dimmer rheostat or the wheel you turn to dim and brighten your dash lights also should turn on your dome light so I think maybe a new headlight switch assembly or rheostat should fix tour problem

Power runs from the headlight switch down to the dimmer switch, then to the lights. Start by checking for power to the dimmer switch when headlamp switch is on. There should be power in one wire and power out 1 for low and the other for high beam.

I'm no expert, I recently purchased a 2003 vibe. I noticed the automatic headlight's sensors can be super-sensitive, about an hour hours before dusk they'll kick my headlights on (when the sun is still shining!), or sometimes when I go into an underpass, and then sometimes they'll realize it's light enough and turn themselves off. This would cause your dash lights to go to the 'headlights on' mode and turn the headlamp signal on, the intensity is controlled by the dimmer/dome light control on the left side of the dash. Check to see if the dimmer is down too far, as this could cause the lights to dim when the automatic headlights turn on. It's also possible that the dimmer is not working, thus no power is getting to the dash lights through the dimmer. Try testing it by starting the car in daylight, then manually turn on the headlights, then adjust the dimmer. Like I say, I'm no expert, just guessing because I've noticed it happen similarly on my Vibe, but my dimmer was at half-way, as I like it for night driving. Someday, I'd like to disable the automatic sensors and DRL.

possibly the dimmer switch on the floor? happened on my 91 half ton dims didnt work but brights did and then they cut out altogether. a little water goes a long way