My tail light bulb keeps blowing. The brake light bit works but not the normal light. Ive been through 3 bulbs in two weeks. I commute 45 miles a day with the lights always on. Ive been using 12v 21/5w. I ride an 07 Suziki GZ 125 Marauder HS.

That happened in my 1999 Buicke Century. Replaced the bulb 4 times kept burning out all within a week. Finally replaced the blinker socket and no more issues.

if you have just replace a tail light bulb check that it was the right bulb as the problem is a dead short
one bulb type has 2 contacts and the other has 1 contact which if placed in the wrong holder will short out the circuit and blow the fuse

Ben, it could be something as simple as a bad tail light bulb or socket shorting out. Check all tail light bulbs, sockets and wiring for damage burnt etc.
Does it blow immediately, or when you turn lights on, or when you use left or right signal, or when brake is applied? this can help trace problem.

First replace the tail light bulbs. The tail light and brake lights are different filaments in the same bulb. it is very possible for the tail lights to go out while brake lights still work. And somtimes the tail lights can blow out together.
If you change the bulbs and still dont work then look for other problems.

If a bulb blows white the gas mix in the globe is incorrect and you probably got hold of a bad batch of globes from your supplier. Try buying your bulbs from another source and see if that works out better.

They are right. The bulb has 2 filaments. One for the brake and one for the running light. One of the filaments has burned out. You only need to replace the bulb. The bulbs are inexpensive. It's just a maintainence thing. Not to worry all light bulbs burn out eventually. It's an easy fix.

Sounds like a short either in the brake switch or wiring .The switch is located near the upper part of the brake peddle.

Check for a short in the wiring, particularly at the bulb sockets connection.
If you have a trailer connection socket, check this also for frayed/damaged wires.
Replace any bulbs that are damaged or suspected faulty.

If you still cant find any problem, remove all bulbs on the tail light circuit - replace fuse - check that is doesn't blow.
Install bulbs one at a time - check fuse as each bulb re-installed- if fuse blows when a particular bulb installed the faulty circuit/bulb is then identified.

The tail lights in my sister's '90s Subaru had a similar problem, and nobody could diagnose it, so she just kept a bunch of fuses in the car. (When she replaced a fuse, it would be a while before it blew again -- days, weeks or even months).
In your case, there's a continuous short circuit somewhere. I'd start with the tail lights themselves -- remove the bulb on one side, replace the fuse, then test. If the other bulb lights and the fuse didn't blow, you know the problem is in the wiring to the bulb you removed. If the fuse DID blow, replace the bulb, remove the bulb from the other light, replace the fuse and test again. Examine the wires carefully looking for nicks or bare insulation.
If the fuse fails whichever bulb is installed (or even if no bulbs!), then the short circuit is probably at the fuse box connectors for those wires, or inside the fuse box itself.
I assume you're talking about the tail lights that turn on and off with the headlights. If you're talking about the brake lights, the test procedure is the same, but don't forget about the high-mounted center brake light, if your car has one.