Hi,
You have a separate 20 amp fuse for the running light. You do have a bad fuse. Look for the fuse box that was factory installed by Ford or GM, and it will also have the turn signal flasher relay in the same box. (you should be able to find it by listening to the clicking while the right or left turn signal is on).
Some auto parts stores sell a kit with several fuses, and also a fuse tester. With the headlight switch on to the "running" light position, you can place the tester on each fuse, the LED in the tester will light up when it is across a fuse that is bad (if there is a load on that fuse).
In my RV, I plugged in my brother's cargo trailer, and blow the 20 amp running light fuse. A quick check, and I found the voltage at the rear bumper was only 9.5 volts to the running lights. With more than 15 lights to run, it can exceed the 20 amp rating, especially if you plug in a trailer with another 16 running lights. So I installed a 12 volt relay to run the running lights, and provide a full 12 volts to the trailer lights.
I plan on installing some 5 LED lights to replace the running lights. Total amp draw should drop to less than 8 amps once I convert them to LED's. I found them at
Aliexpress.com Do a search for 5050 LED and you can find them.
Fred.
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