the 'Harness' I was thinking was the wiring from inside the box to the female port to plug into the sled
Ah. That's the ISO adapter, yeah. Doesn't do you any good at this point.
The sled is probably still installed in your late brother's car. Any idea where it is? If the car was sold, its buyers would have no use for the sled and it would have no value to them. However, to the empeg owner, the sled is priceless as they are very difficult to replace. I'd see if I could pursue tracking it down if I were you.
I will try to install the upgrade when i find a serial cable around, hehe.
Remember that it has to be a null-modem cable.
The pins on the cable that are used are pins 2, 3, and 5. (RX, TX, and Ground.)
Pins 2 and 3 are "crossed" on a null-modem cable, they are straight-through on a regular cable.