The OE park light connector isn't the best design and causes issues down the line. All the headlamp lights ground thru a single earth wire for each side so if the dipped beam works then shouldn't be an earth issue.
The parker earths thru a bare copper wire on the bulb connector to the headlamp bowl. Check you get 12v on the parker bulb connector and if you do then means the little copper wire isn't earthing properly to the bowl. To check just use a spare length of wire attached between a good earth and the parkers earth wire. if it lights then defo the earth contact.
Take a look at this diagram
Samba 1970 bug wiring diagram
Now IIRC the white wires are for the main beam but all you do is check the fuses of eaither the yellow and white and whichever has 12v when dipped is on then it's the other 2 fuses for the main beam. turn main beam on and if no power at the fuses then more than likely the relay is dodgy or could be dirty relay terminals.
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