I did a quick Googgle and found a lot of comments that this was difficult.
For example, on
one website, I found this comment - "As a result of the way that Dapol designed this model there's no simple way to convert the 66 and retain the lighting. The lights are connected to the circuit board so closely that it doesn't seem practical to break the connection and rewire without damaging either the LEDS, the shell or both. The circuit board is helpfully mounted so that all the tracks are facing into the apex of the roof, so they can't be intercepted, and because of the connections to the LEDs the board can't be removed."
On
another forum, I found that "The only major problem is the current DAPOL Class 66, which is a nightmare. However, I understand DAPOL are now working on a replacement chassis which will be DCC friendly".
From the sounds of it, you would be better off swapping this model with somone else who has the DCC Ready version, or just buying the new version and selling the old one to someone who is not interested in DCC. If this is your first attempt at DCC, it would be a shame to be put off for life by a bad experience as a result of this "design error" from Dapol.
I was very lucky, as my first decoder was fitted to a 00 scale class Q1 from Hornby and I have read since that this is one of the easiest to fit in the Hornby range. It was the SR livery, so there was no funny speedo cable to remove, just one screw to and lift the body off, plenty of space inside for any sort of decoder. DCC ready with the standard socket. It took longer to figure out how to set the address (from the default of 3), than to fit the decoder. It's been downhill ever since!