This is all good stuff and since the topic of the DB10 relay and its mysteries comes up from time to time I thought a diagram that shows exactly what is going on inside the box might be useful.
The box contains a dual relay (which is just two relays joined together electrically), one for the left side and one for the right.
When the direction indicator switch is not operated neither relay is energised. When the brake pedal is pressed the stop light switch is closed and the circuit is made in through tag 5 and out to the rear lights through tags 3 & 7. This is the red path on the top diagram.
When the direction indicator switch is closed one of the relays is energised depending upon whether left or right is selected. I’ve shown the left side energised in the diagram. This relay changes over and makes two circuits, one from the flasher unit directly to the front indicator on the left side, the other breaks away the circuit that was from the stop light switch and then connects it to the flasher circuit. This is shown in amber on the second drawing. The left front flasher lamp and the stop light filament in the rear light are now connected to the flasher unit, the circuit coming in on tag 1 and going out on tags 2 & 3. Current flows in this circuit and the will lamps flash. You can see that if the brakes are pressed at the same time the right hand brake light circuit is not affected and will be steady not flashing.
Exactly the same is true of the right hand flasher circuit except that the other relay gets energised when the right hand turn signal is selected. Note that the ground connection for the relays is via the case of the DB10 and this must be correctly grounded for the relays to work.
A quick test to see if you have the flasher wiring correct is to connect a jumper wire between tags 1, 2, & 3. Front and rear left hand flashers should flash if the rest of the wiring is OK. Jumpering tags 1,6 & 7 will test the right hand side flashers in the same way.
Eric
Shropshire, UK