All running lights work and so does the horn. Fuse is good. Need to check brake switch or ground (g503) but before I check those, is it possible to test the output of the bulb holder using a digital multimeter? What is green, red and black wires?

 

Forgot to mention the brake lamps will be plugged and pedal stepped on. Picture shows my extra set for reference only

 

The Grn/Wht wire is the positive (+) power wire for the brake light. There should be power at this wire with the brake pedal depressed.

The Blk wire is the negative (-) ground wire.

The Red/Blk wire is the positive (+) power wire for the tail light. There should be power at this wire when the parking light or headlights are turned on.

You can use a multi-meter set to the voltage setting to test the terminal contacts. Red meter lead to the Grn/Wht wire and Black meter to lead to body chassis (preferably unpainted area or bolt on body). You can also touch the meter's black lead to the black wire terminal, assuming the ground is good there.

 

Thanks for the reply. With the red meter lead I'm getting 12 volts coming from the red/blk and nothing from grn/wht. I used the ground from the bulb holder with the black meter lead. Since I'm getting good contact for ground, im going to assume ground (g503) is good and I'm left with the brake switch to test