Switch Wiring Question
 

Edit: decided to tap the tail light fuse and forget about getting them to dim.

I'm getting ready to install some LED lights and OTRATTW switches on my 88 Pickup. The switch has a positive and negative that is used to turn on the backlight on the switch.

When I did this on my 4th gen 4runner, I tapped into the OEM dimmer switch. When just tapping the positive, the switch dimmed, but opposite of the other dash lights. I had to run both positive and negative to the OEM dimmer switch to get the new aux switch to dim correctly.

I was expecting to do the same on my pickup. I don't have the switch yet, I'm just using a multi-meter to figure out which wires are which. On the factory dimmer knob, there is a red/black (+) and white/black (-) wire. Either way I connect these (only connecting + or connecting both + and -, it seems to be working opposite the dash lights. With the dash lights on the brightest setting, I barely get any voltage, but with them on the dimmest setting, I get full voltage.

What do I need to do to get the backlight for the switch to come on only when the headlights are on and also to dim with the other dash lights?