T25 Dimmer switch Burnout

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Hi, new to the forum, have had a T25 Multivan for a year or so.
I replaced a faulty dashboard cluster with a Vanagon set from the US but the Headlight switch had a different pin arrangement so i used the one from the old cluster. ( The difference is 10 pins on the old one, only 8 on the US version and the two biggest pins are on a slightly different pitch which doesnt fit the loom.) All has been fine for some time. But, I used the for lamps and left them, and the lights on. Fortunately I notice after about 10 minutes. When I opened the door to turn the lights off, I was hit by a overwhelming smell of burning plastic. The dimmer switch was burning a dirty great big hole into the side of the switch. I dread to think what would have happened if I hadnt seen that the lights were on.

anyway, does anyone have an idea why this would happen and where I can find a 10 pin switch?

thanks
Ian

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Quick Bump for Sunday evening....