Dash not reading

[SprinterTRD]

Unfortunately regardless of what your tuner said its still worth checking for your own sanity!

Either the wiring is wrong or has a problem or the setup is wrong, assuming the hardware hasn't been damaged during the swap process.

If you are absolutely confident everything is correct then you need to get a oscilloscope and start measuring the waveform's from the ECU to dash.

[Kdog]

Yes, agreed. It looks like a maybe a port issue on computer. An oscilloscope isn't going to happen today. Are there any checks I can do with a multimeter?

[SprinterTRD]

Not really, you need the scope to see if the RS232 messages are output from the ECU.
An alternative is to use a terminal emulator on the PC, connect the RS232 from the ECU to the PC and see if you see ASCII characters appearing in the terminal.