Tail light/fuse

Don't know if anyone can help me on this. I have a 1998 GMC truck (http://www.carforums.net/showthread.php?p=734803#) and when the tail light goes out, it gets a fast signal, meaning the clicker speeds up to let me know it's out. This happened and I had the wife look at it and she said the brake light was working but not the blinker. A few days later, I went to fix it and had my son check the brake light again and it was now also out. I removed the housing for the bulbs and all the bulbs looked OK (to me, meaning no broken wires). Would this be a fuse problem, and if so, where is the fuse? I opened the fuse box under the hood on the left and there is only a "brake light relay," which is a square, white plastic plug with no fuses.
ANy help appreciated.
Thanks,
gb

do you have a trailer plug? if so check all the wire and plug for shorts ect.

Start with replacing the bulb anyway. Looks can be deceiving.

twice I've had to do the taillight circuit boards in my classic k2500 sierra w/156,000. The circuit boards are cheap and don't forget bulb grease