I've been making cassette tape loops recently, which necessitates disabling the erase head while recording. Constantly placing and removing a tiny sliver of tape and aluminum foil over the erase head got tedious quick. I've added 4 switches to turn on or off the erase head for each of the four tracks.
Here is an article that goes into more detail. It only posits the wiring though, and after some more research I learned that ideally there is a dummy load so the voltage that would've gone to the erase head doesn't go somewhere else, potentially messing with biases.
SO. Ground wires to the erase head (black on the Fostex X-12) were left in place and the SPDT switches are also wired to a separate erase head that is taped inside the case. Here's my crappy diagram of the erase head wires: