Check grounds, aye.

In my experience, even better than WD-40, use cleaner electrical contact cleaner designed for that purpose on switches and similar, such as CRC 5103 Quick Dry Electronic Cleaner. Walmart and FLAPS should have it.

1947 2nd-Series GMC FC152 3/4-ton
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1953 Chevy Bel Air Sport Coupe Powerglide