[Peter Milnes]

The overdrive relay is the rightmost one on the second row of relays behind the fuses at the bottom of the centre console (remove ashtray for access). As the lamp and the overdrive solenoid are fed from the relay contacts, then if the lamp is on then the solenoid is engaged and the overdrive is off. It may be that the relay is loose in it's holder but it is far more likely to be in need of resoldering of it's internal connections or replacement with known good one or new one.

All the best, Peter.