Why does my tail light fuse keep blowing ?

Check the tail light bulbs & marker light bulbs.

Try disconnecting both tail lights replace the fuse and see if it blows...if it don't connect one at a time and see what happens...also check you third brake light...

If you are replacing tge correct fuse and its blowing then there must be a short circuit, I have jnow bulbs to cause this sometimes so I would remove bulbs amd check or just buy new bulbs as tgey are wuite cheap, replace the ones in the tail lights first, also is this just when the lights are turned on? Is the fuse ok when braking? Hope this helps

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

On some models the light switch it defective increasing the volts in the tail and brake circuit.

you got a short,,,sometimes the light bulbs go bad but still work if that makes sense,,,,the dual fillament for the park lights and the brake lights touch and casue a blown fuse,,try replacing all the tail lights bulbs with new ones,if this dont work,,check for a bad wire

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.

I would look for a wiring short in the trunk at the taillamp area, or, inspect the sockets for the taillamp bulbs for corrosion. In any case, you have a short circuit in the trunk area, also check the wire that comes from the body & goes up the trunk hinge on the driver side; that wire may be chafed, and it may be shorting out. Hope this helps; please get back to me if you need any further assistance.

First ensure the bulbs are fine.

Check and replace the fuse in your fuse box.

If it keeps blowing after replacing the brake lights fuse, then there is a short on the wires, you need to check wiring.

If fuse is ok and there is no brake light check wiring to the lights, there is a broken connection or an open wiring to the brake lights.

Note: brake lights are usually not dependent from tail lights, they have separate wiring.