My unit is a riocar, not an empeg so I assume it has the grounds. Regardless, it worked when I connected my laptop to it through the sled serial port so I assume it must have the grounds. So, if I can connect it to my laptop without a null modem adapter then to connect it to the wideband (which also connects to a computer without a null modem adapter) I must have to use a null modem adapter to swap the RX and TX so they can talk. But, it doesn't seem to work. I'm going to go out in a bit and try connecting it to the wideband in the car while it is on AC power to see if the problem is something to do with the wideband when it is installed in the car.
The strange thing is that I can talk to both of them with my laptop so it seems to me that they should be able to talk to each other unless there is something funky going on with a ground loop or the voltages for the wideband being out of spec...
I'll let you know what I find (if anything).