First remove the plug-in relay and two little screws and carefully prize open the back of the assembly. This needs doing carefully working your way round a little at a time:
Then prize out the PCB - there are some pointy parts to the PCB locating it in the base moulding around its edge:
The PCB will pivot upwards and reveal the switch innards. Don't loose the springs for the horn contacts:
The two white plastic parts and the left hand lever contain pairs of these spring loaded roller contacts - don't loose any!
Clean up all the PCB contact areas with a degreaser like IPA then clean up any burnt or marked contacts with some light abrasion like this glass fibre brush pen and a jewellers screwdriver. An ink rubber will also work well:
These are the indicator contacts cleaned up with the wiper in the bottom of the picture. See how it's eaten away at the sides? That comes from not pushing the indicator stem all the way. After a while the wiper wears but leaves the centre intact which rides high on the nylon insulator in between the PCB contacts - then it arcs badly.:
I'm not sure what was causing the problem but these two little clear pointed rubber end stops (one under my finger) had split and fallen off and may have lodged in the works - or it could just have been dirt and oxide from all the arcing getting in the way. I glued the split end stops in place with Superglue.
The light switch can be pulled out. I filled it with switch cleaner spray and switched it lots to clean the contacts before tipping out the excess.(http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=278786&N=401 Also available from Maplins)
I noticed this red wire's insulation was rotted and needed sleeving. The wire is a multi stranded extra flexible sort and best retained for use.
I used petroleum jelly to lubricate things again - all the swivel joints, bearing surfaces and importantly the switch wiper and PCB contact surfaces. It's important to exclude air from the mating contacts, otherwise arcing in air will break down conductors and insulation forming carbon and oxides. I only had petroleum jelly which will work but there are better alternatives.
(e.g. http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=279493&N=401)
Do not use silicone grease this will form silicone carbide and abrade the contacts.
Grease or petroleum jelly also holds those roller contacts in place. You can see the white cancellation device here - only very slight lubrication on this or it will get stuck:
This is the underside of the indicator stem with the white roller position stop lower left. You can also see the pivot point for the stem and the little walled area that the cancelling device sits in (below my thumb). You need to reassemble by pushing the roller against its stop wall and lower the stem pivot into its hole in the moulding and tucking the wall over the cancellation device and under its back support. A similar manipulation needs doing for the wiper stem.
Both stems need holding in place to avoid the stem position rollers springing up whilst the white plastic parts (with the contact rollers) are put into place. There's a little pivot hole in the main housing for the lower end of a long shaft at the back of the white part - same on the other side:
You then need to press the PCB back into place, making sure that the relay contacts don't get bent and the stem's position rollers don't pop up. The PCB snaps into place. Excercise the arms to make sure it all moves correctly but hold the PCB tight in place whilst doing this. It took me two goes as one of the roller stops fell out!
Pop the two flying horn wiper contacts down their holes making sure the wire exits the hole via the groove at the side and then insert the springs.
Then it's just a case of putting the top back on and screwing it all back together
