I have the same year, make and model and I just discovered this problem a few days ago myself. I'm wondering what your result was. I've removed the front hand brake lever, mashed the switch(which seems to spring back and click) with no results to the brake lights. Is the back switch hard to find and check? If anyone has suggestions, I'd appreciate them.

 

Did you try jumping out and/or removing the switch to see if it changed taillight brightness? When I bought my '04, it had the same problem which turned out to be failed front brake switch, which left brake light on constantly. Really confused me until I figured out how it works.

 

probably BOTH brake light switches are bad.. most people don't preflight their bikes ( I don't ) and if a switch quits you wont notice it and neither will your riding buddies so long as one of them are working..... unplug the switch and put a voltage tester on it
and see if you have 12 v once pedal is depressed or lever pulled in..


being an 03 I believe your rear brake switch also is the screw in type like the unit on my friends 05.. if so we found that the duralast RB401 from advance auto is the exact same switch and I think it cost him a whole $8.50

 

Did this just happen out of the blue or was some work done somewhere that could have compromised the integrity of the wiring?

My wiring diagrams for a 2003 FXDL show two wires, a red with yellow trace and a orange with white trace wire going to both front and rear brake switches. The orange/white wire is the power input wire and comes from the accessory fuse and the red/yellow wire is the output wire and goes to the tail light. The red/yellow output wire from the switches has a common tie point somewhere in the wiring harness, probably under the seat or at the rear switch.

Pull the connector off of the rear switch. If it has two contacts, jump them and the tail light should work. (If it only has one contact, come back and let me know and I will look at my son's 2004 FXDLI in my garage that we are working on and I will see what gives.)

If the light works, the switch is bad...if it doesn't, then you will need to check for power at the switch (use a test bulb) and if that is good, then check the wiring from the rear switch to the tail light. There is a connector at the lower front of the rear fender and another one going into the tail light. Look those over real good because one of those might be corroded or have a broken wire or bad contact.

Let us know how it goes...Good luck!