I think we need more info to make this analysis...I assume it is not the "non switched" power that keeps the clock running that is bad...so that leaves the "switched" power or the ground if the radio shuts off...this would mean that the tape or CD player would stop and reset and the lights would go out!
On the other hand, if the player continues to run and the lights stay on but music stops, it is likely to be somewhere in the speaker wires.
If you have the power hooked up to non-switched source (purple wire) the problem could be in the dash connection in the harness...one reason Lucas is known as the "Prince of Darkness"
If you would like to find an alternative "switched" power source there and don't have fog lights, the fog light wires are "switched" and can be used...they are Green/brown I think (I am at work now and doing this from memory) on my 74B which i am current redoing the console on.
If you have problems or questions, email me this weekend while I am at home and I can fax you a wiring diagram if you need it and/or more closley check my harness...which is currently complety exposed on mine until I get this carpet job done.
Good luck!
Bruce