I have a 2008 Ranger Sport. 4.0L engine. The dash illum. lights are getting dimmer and dimmer. I tried moving the dimmer dial back and forth several times. No help. I replaced the dimmer switch. I'm wondering if there is a dimmer relay or module? All of the lights are working, I just can't get them above dim.

 

2004 and up Rangers have the HEC instrument cluster, and the back lights do look to need clusters micro processors pass-thru ground to work right

Do the radio and climate control back lights get brighter and dimmer using the control wheel?
If they are also not working like they should then it has to be dimmer module

Diagram below
When Park light relay is activated it sends the 12v to the dimmer module, so if parking lights are bright then it should be getting full voltage
You can check fuse 1, 5amp, for corrosion and make sure it shows Battery Voltage with Park lights on
The PWM(pulse width modulation) in dimmer outputs 5 to 12v depending on selection
It goes out directly to radio and "other" back lights, which are directly grounded
The cluster lights also get the PWM voltage but are not directly ground, they are grounded thru the micro-processor(lower left in diagram), not sure why that is

Google: Ford HEC cluster

There are how-tos to explore the different parts in this cluster

 

I'll start on this as soon as I get the ,91 f800 repaired. That one is higher on the repair list