Hot skinny on the Fostex D5 remote:
The remote logic is ACTIVE LOW, TTL Logic, with the following assignments, clockwise, from the right most pin:
Pin 1 GND
Pin 4 Search ID >>
Pin 2 Stop
Pin 5 Search ID <<
Pin 3 Play

These logic signals are converted into an analog voltage level for transmittion the the signal processor or the Main PCB. There are no assignable analog functions, nor can we reassign the existing ones.
All is not lost. The Front Panel switch matrix is also an analog voltage level encoder which consists of resistors switched to Gnd.

The REC. Enable function may be connected to the REMOTE as follows:
1) Remove both the Remote/Optical PCB and the Balanced Digital PCB. (The ribbon which connects them is soldered fast)
2) From the Remote PCB; Remove transistor Q3704 to disable Remote ID Search>>
3)Attach a wire to Pin 4 of the Remote connector.
OR You can disable the Remote ID Search<< by removing trransistor Q3703 and attaching your wire to pin5 of the Remote connector.
4)Reinstall circuit boards
5)The other end of the wire is soldered to the "arm" side( -o\o-) of the Front Panel REC Switch, S1506. See silk screening on Front Panel PCB.
6) If you intend to operate several machines in parallelI: I would install common switching diodes, Anode toward the D5, on the remote plug to isolate the machines from each other.