OK, I think what you are looking for is like a relay that when the coil is pulled in it will have a signal coming in that varies and the output varies but you won't have a big load on the input side as there is isolation between the input and the output.

What you might be looking for is a power supply circuit that will match the output voltage to the input voltage but draw the current from a main power bus. So the power supply will need an enable line that will activate the power supply. Then when the power supply is activated it will match its output to the voltage on an input line from the circuit that you are monitoring.

I know how to do this in an electronic circuit but I do not know of any ready made modules that do this. Let me do a little looking and I will see what I can find for you.

 

OK, the basic circuit that you want is a voltage follower. It is a circuit that will put out the voltage that is present on the input side and some of them that I found in my Google search can put out 1/2 amp of current.

You will need to tap into the vehicle battery power as a power supply for the circuit, the vehicle ground and the circuit that you are trying to match for voltage. Then there will be an output that is referenced to vehicle ground. Whenever power is on the circuit from the battery the output voltage of the circuit will be very close to the input voltage of the circuit. You may want to put a relay in the power supply line to turn the battery power to the circuit off when the truck is off so there is no battery drain when the truck is off.

I did my search on google for High current voltage follower. They are simple to build and should be able to be scaled up to make more than 1/2 amp of current.

 

You da man! :bow:

Now the search for an appropriate spec ready to use module begins!

The few pieces of hardware with the label of voltage follower have little to zero information or description. Its odd :confuzeld

 

Im going to 86 this project and just switch over every bulb on this circuit to LED. I didnt realize until recently how easily accessible all the bulbs are in...everything. Aside from main instrument panel its just the A/C control panel(s) and the door lock and window switches, all of which the bulbs easily come out of the back with snap ring pliers.

Switching them all over to LED will allow plenty of headroom to have additional [LED] lights for future additional switches and gauge pod.

 

Won't LEDs have dimming issues without an LED dimmer circuit?