Thanks marria01.

I have pulled the system out of the car and have it back on the bench now. With the tuner module plugged in, I have just checked continuity from the tuner PCB to the sled connector and everything checks out fine, all of the pin numbers and wire colours are as per the instructions and match the tuner connector colours and pins from the Empeg itself.

The only thing I can see at the moment that could be a problem is that the dual op amp shown as LM258N on pca's list is marked KA258 on the component. I presumed, when I was going down the checklist, that this was just a generic part with different codes from different manufacturers - is this right or has a rogue part slipped through?