If no flow control or handshaking is being done, then I would only connect the 3 wires. 2-3, 3-2, and 5-5. I am not sure why pin 8 would need to be available on the empeg if no handshaking was used.
Ah, just looked on the developer info page. It says pin 8 is being used as the headlight sense. In that case, I would not hook up pin 8. If you hook up pin 8, it then is swapped in the empeg supplied null modem cable to the RTS signal, which is an output from a DTE.
If the RS232 connector on the sled is swapped, and therefore a DCE, then it is strange that the headlight sense would be tied to pin 8, which is an output on a DCE.
Perhaps it means that headlight sense is tied to pin 8 on the RS232 connector in the empeg itself. That would make more sense, as that is a DTE, and pin 8 is then an input.