First check the wiring outside the device. Verify that nothing touches the body of the car or some other metallic or conductive element. Inspect the electrodes inside the device. I've been asked if they should be touching each other. Well yes and no.
Sometimes there are two wires in each electrode, sometimes only one. Each electrode, which is the stainless steel wire, or a pair (sometimes even triple) of wires, coming out of negative or positive terminal (the bolt with the wingnut on the lid) and spiraling down, these should be in one piece (i.e., not broken or visibly damaged) and spiraling at intervals of between 3/8" to 1/4" from the other electrode.
If the electrodes are too close, separate them as shown and glue in place with Plumbers Goop. If an electrode is made up of more than one wire, its wires can be touching or not touching, as long as none of the wires are too close to the wires in the opposite electrode.
Some manufacturers use braided cables, so each electrode has 10+ wires. That's ok, as long as they are made of durable metal, and not touching the other braided cable.
Another major reason for the fuses to blow is that the fuse rating is too close to the current being consumed. For example, if you're measuring 3.25 Amps and using a 5 Amps fuse, the fuse will blow during the first or second trip around town. Why? That's because the current you're measuring is probably at idling, but on the road there may be peaks of high voltage exceeding 14 volts, and with heat and motion, the current may peak momentarily above 5 amps and there goes the fuse. To protect a system consuming what seems like 3.25 Amps, for example, you should use a 8-15 Amps fuse.
If you are using a metallic lid over the Electrolyzer, watch for shorts caused by the lid touching the electrodes or the connection terminals.
Otherwise, use common sense and standard automotive electrical standards. Watch for broken wires, wrong connections, etc. Consult an auto mechanic - or better yet an installer of car stereo.