Vintage Cars: MGB ignition, light dims, input welcome

Question
When turn signal, horn stalk or wiper stalk is activated the tach drops to zero, fuel pump stops and ignition goes away. I then turn off key and then turn back on and pump etc are back to normal. 1970 MGB-GT Sometimes it does this without any stalk interaction.   Suggestions please  

Answer
Goodness, it seems that 90% of my questions these days are about MGB electrics! As usual, the symptoms are baffling, and note that the latest MGB I ever worked on was about 1968! Really only one suggestion, and this may well not turn out to be so; but try it anyway. (You might also try other experts, e.g. Fitzcharles; and I have found http://www.mgb-stuff.org.uk/ exceptionally good on all sorts of details, FAQs etc.)

It sounds like everything that goes through the ignition switch gives up the ghost when you put a load through it. The fuel pump is particularly notable, since it's almost a direct connection from the switch (I'm not sure it's even fused). So I suspect the sickly behavior (kills the car!) is true when you switch on anything that draws significant current through the switch, i.e. anything that is live when the ignition is on. So first suspect the switch, and its connections, both ends, power in and out. For each end, check whether a 12V testing light dims when you put a load on, e.g. the flashers. If my switch theory is right, the main input will remain fine, but the output will be way down. If not, take the problem a step further, to the fusebox and fuses. And also do some cleanup on the fuses and clips with sandpaper.

If the switch is dud, a new one would be best.

Good luck!

JJ

PS To other readers...other input welcome!