So on my '73 F100 the rear running lights are on the brighter filament of the bulb, and only the upper two light up. When I installed a new brake light switch (it didn't have one when I bought it), nothing happens with the lights on because the bright filament is already lit. As a hokey way to fix this, I ran a new wire from the brake light switch back to the lower rear lights, and replaced the lower lights with some brighter LEDs. It works wonderfully with the running lights off, but when I turn them on the LEDs lit up too! It turns out when you turn on the running lights, it sends power to both terminals at the brake light switch, so now the switch does nothing and all the lights connected to it turn on with the running lights. Does anyone know how to remedy this? I'm tearing my hair out here. Thanks in advance.

 

bulbs are in backwards

 

I don't think these sockets will let the bulb go in backwards, the two grooves in the socket for those little nubs sticking out of the bulbs correspond to the depth of the nub.

 

Sounds like a wiring problem

Here is a diagram. Post #3 in this thread

 

Right idea, but as Champion pointed out the socket will not allow the bulb to be installed incorrectly. However, if the bulb socket has been replaced, it could have two wires switched or same problem somewhere else in the wiring.

 

They will go if you force 'em. I've done it back the day when I was younger and stupider.