how to include fan light in dimmer system

I added an alternate radiator cooler and fan for my tranmission coolant and also added a manual on/off switch for the fan.  I wanted to know that the fan was running so I added a 12 volt indicator lamp into the power input circuit for the fan.  Whether the fan comes on automatically via the thermostat or I manually turn it on this lamp will illuminate, the problem is that it's too bright in the evening but it's perfect for daytime driving. 

My question, can I somehow integrate this lamp into the dimming circuit for the headlamps system?  Or do I need to add a manual dimmer switch, which I really don't want to do if possible.  The lamp is a simple 12 volt DC two wire lamp, one to ground and one positive. 

You could set it up through a relay to give you "hi" and "lo" settings fairly easy.

Take the output coming from the switch/thermostat and wire it to pin 30 of a relay.  Wire pin 87A directly to the lamp.  Wire Pin 87 to the lamp through a resistor (the resistance value of the resistor will determine how dim/bright the light is). 

Wire pin 85 to ground and pin 86 to the parking lights.

When the parking lights are off the relay won't be energized and the light will work just like it works now.  When the parking lights are on the relay will energize and now the lamp will be fed through the resistor, causing it to be less light.

This will work on both LEDs and incandescent bulbs, but the resistor value needed may vary drastically between the two.

Before you try to tie in to the dimmer circuit you will need to verify if it is a + or - acting dimmer switch.  If you try to dim the + of the bult with a - dimmer (or vice versa)  your light will light up backwards from the rest of the lights.

Okay, you guys are great for responding so quickly.  I have two last questions.

Based on the above recommendations, if the lamp is on during the sunset hours and I switch on the headlamps, will the indicator light dimm?  I understand the wiring discussed above but it sounds like it will either be dimm all the time.  If I am misunderstanding just tell me so.

Second question, I'm not an electronics technician, what resister would I use for this scenario?  Considering a typical 12VDC green indicator lamp and a 12VDC relay.  I would like to dimm the lamp by 50% only when I switch on the headlights. 

Disregard my last question, like you already have, I studied the relay section and figured out that your recommendation will work.  I'm still wondering about which resistor but I've tried a 1watt/100 ohm resistor and it seems to do the job.

Regards,

BBusbee

There are two ways you can get a close number for the resistor -

1. Use an Ohm meter and figure out the resistance of the bulb you are using.  You can then use a resistor of double that number to reduce current by 50%.  Say you have a 60 ohm bulb at 14.4vdc = 240 mA of current.  If you put a 120 ohm resistor before it you will reduce the available current to 120mA, which will in turn dim the bulb by 50%.  Keep in mind that light produced isn't linear to the eye, so you may need to adjust the value a little bit.

2. Buy a 1000 ohm pot and use that.  You can "dial" in the amount of light you want and then either A. measure the resistance of the pot and buy a resistor close to the measured value or B. tuck the pot away and forget about it.