Thinking in type.....

Or could it be an issue with the 'hot' feed from the ignition to the lamp? It's dropping voltage or finding earth somewhere which is causing the lamp to light. The 'hot' feed to the alternator lamp shares an ignition feed with the wiper motor.

"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10