Try disconnecting the tach light from the dimmer circuit temperarily. If the dash lights now work, then something is connected improperly with the tach light. I've found the best approach is to avoid connecting grounded backlights to the dimmer circuit. I've got enough other un-lit gauges and switches now that I am planning to add a second dimmer rheostat to supply all those items. This one will be wired to work off of the power side: 12V-dimmer-backlights-ground.

r.c.brown@ieee.org
TruckEditor@tlca.org
Project: 4Crawler
4Crawler OffRoad - 4x4Wire discount code