I bought a 2003 ford f150 four wheel drive. It has two bulbs in the back on each side. One is a two filament and the other is a one filament. I went to the auto parts store. They said they both take the same bulb. 3157 , But the ones that are in there are a 3157 two filament and a 3156 single filament. Why is that, and which ones are for back up and which ones for brake and running lights.? thank you very much.

 

parts store counter monkeys have the IQ of a dead cockroach.
3157 is the brake/ blinker/ running light bulb. it goes in the top hole with the red lens.
the 3156 is the reverse lights, and goes in the bottom hole with the clear lens.