Possible by using two steering diodes. I believe 1N4001 diodes will support 1 amp. Should be more than enough for a small indicator lamp.
Jim, one power from relay ( black/yellow), other from fan.
Tom

I am attempting to use some of the CAT wiring to turn on the CAT light ( it's a 75 ) when the oil cooler fan comes on.

CAT light always comes on at start up, then goes out when engine is started. The CAT sensor is not Kinstalled. My thought is to use the wiring that goes from the rear trunk to the dash to indicate the fan is on.

Here's a wiring diagram of the harness in the rear trunk. There is a relay involved.

I'm also thinking it will be easier to run a wire from the fan through the tunnel and jump into the CAT light connector too.

** I previously referred to this as the EGR circuit, but was mistaken. This is the circuit found in the rear trunk with a pair of wires going to the sensor

If the EGR indicator is nonfunctional disconnect its wire from the dash and attach your fan wire.

I installed an LED into my oil temp gauge to indicate when my fan is on.
Click to view attachment

Had me baffled for a moment, then I decided you meant "EGR", not "ERG". Since you no longer have an EGR, why not just wire the light directly to the source you'll be monitoring?

The Cap'n

I like that better. What's involved in drilling the hole...

I bought the LED at Frys Electronics. I was sure to get an LED with a bezel. I took the gauge apart and drilled the hole by hand (it came out a little lower than I wanted). The needle clears the LED. I used a variable resistor on the LED to control the brightness .
Click to view attachment
Click to view attachment

Are you making 1 idiot light illuminate from 2 sources? It's a little unclear, your intentions. If so I agree with your simple schematic.

I did this same thing with my custom EFI system. Various signals can turn on the "check engine" light.

Mike,
Yes, two different power sources to light one lamp. It is for Jim in post 2.
Tom