Off and on (for a few years) the right front parking light will not work. After 1-6 weeks it works again; then it's off again for a period of perhaps 3 days to 2 months. The turn signal light always works. Sometimes if I pound on the light lens, it comes back on; thus I don't think that it is the headlight switch. So, does anyone know where the ground wire for the parking light is? AND, does anyone have any suggestions as to what to check? I switched the bulb with the left side, and switched the ceramic socket adapter (?) with the the left side and still the same problem, while the left side always works. Going over bumps doesn't help. So.....help!

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback