Headlights not working

I've got a problem - I own a 89 Jeep Wrangler YJ - the tail-lights work, parking lights work, side markers work - they all light up when you turn on the parking light switch as well as all light up when the headlight switch is turned on. The headlights however, will not turn on.

There is power going to the headlight switch, there is power coming out of the headlight switch, to the dimmer switch. The dimmer switch has been replaced. The headlights do not light.

If the battery is disconnected, and an auxilliary power source is put on the wires within the connection block that goes to the dimmer from the headlight switch (the wires that control the low and high beams), the headlights light up.

If anyone could steer me in the right direction as to where I should be heading to fix this problem, I'd greatly appreciate it.

Although I'm not the electrical expert here, it sounds like you've already isolated the location of the problem.

Since putting voltage on at the connection block lights the lights, and you have voltage coming from the headlight switch, which then runs to the connection block....the problem has to be between those points. Check where the wires run through any body parts/firewall. A small worn spot in the insulation where it rubs on the body would be enough to ground it out.