The problem with the flashing LED is not the missing ground problem.

The sled is wired up as half DTE and half DCE:
There are 6 wires on the sled:

TXD on the sled is connected to RXD on the DB9
RXD on the sled is connected to TXD on the DB9

i.e. the sled is wired up as DCE so you can use a straight trhough cable.

BUT

The reset of the connections appear like a DTE
DTR on the DB9 is connected to the amp remote output
CTS on the DB9 is connected to the hedlght sense line
DCD on the DB9 is connected to the mute line.

and theres' ground.

So for some reason the sled port is half and half. If you connect a 9 wire straight through cable to the empeg you will connect your PCs DTR to the empegs DTR (a bad thing) and the empeg will go into this flashing rather than pulsing LED mode.

Don't ask me why they did this. Especially since the sled in the car is more likely to be connected to a device like a GPS/GSM modem which has a DCE connection. The built in connector on the empeg should have been the DCE connection IMO so you could connect with a straight through cable.

The sled issue is fairly easy to correct since you can pop the pins out and swap the TX and RX lines which is what I have done.