2000 F150, I would like to wire my fog lights to come on when the engine is running, to use as running lights. Could I use alternator output to trigger a relay? If so, which wire do I connect to? The other alternative would be a vacuum operated switch, which is quite expensive.

 

there are multiple key on fuses to tap into (radio power is a common one to trigger a relay).

 

This is probably the easiest to tap into, simply because it’s in the dash and easy to work with and hide the relay under the dash away from any abuse that could happen in the engine bay like heat or mud or water. Just make sure you put the wires into a loom to protect them

 

OP, the radio power is not a good choice. You want to find a power source that if hot in *run*, not *accessory*. As mentioned, there are several sources available. It's been awhile since I chased that down. If someone else doesn't chime in sooner, I'll try to look at the wiring diagrams tomorrow.

 

I was thinking about connecting a relay to be triggered by the alternator output so the light will only be on while the engine is running, but I'm not sure which is the output. I don't want to open any other wires. I did this a long time ago on a 53 Ford. Worked fine.

 

Seems a bit silly to me, because how often do you have the key in "run" and not have the engine running, but you could trigger off the fuel pump relay. You'll get a 3-second on-time at prime, then the lights will only be on when the engine (and therefore the pump) are running.

 

There is a plug under the removable piece on the top of the dash passenger side for daytime running lights. I use it to power my dash cam. In my 99 xl.