I purchased 3157 style (blade) LED replacements to upgrade my 2001 Ram 1500 and 2001 300M rear lighting. I'm having the same issue with both vehicles.

When replaced with LED's, I have functioning brake lights (and turn signals on the Ram) on both vehicles, but the running lights do not work on either car. If you flip the bulbs to reverse polarity, then the LED's don't work at all. Is the Mopar harness not compatible with LED's? Is there any fix?

Thanks!

Usually the issue with LED's is with the turn signals, not the brake or running lights. The LED's don't draw enough amperage to cycle the flasher - you have to use the correct flasher for LED bulbs. Also, I didn't know you could flip the bulbs if they were used for both the running lights and the brakes - there is a tab on one end to make it directional. They can be inserted either way in the back-up socket, or the turn or brake only sockets - at least thats how the rear sockets are on my 02 Intrepid and 99 Dakota.

I see why the turn signals work on the Ram - they usually have electronic flashers that aren't load sensitive, due to trailer lighting. The 300M probably has a conventional flasher, and would need to be changed. I don't know why the LED running lights don't work if the regular 3157's do.